Align atomic storage

This series adds the __aligned attribute to atomic_t and atomic64_t. It also adds a Kconfig option to enable a new runtime warning to help reveal misaligned atomic accesses on platforms which don't trap that.

Some people might assume scalars are aligned to 4-byte boundaries, while others might assume natural alignment. Best not to encourage such assumptions.

Moreover, being that locks are performance sensitive, and being that atomic operations tend to involve further assumptions, there seems to be room for improvement here.

Pertinent to this discussion are the section "Memory Efficiency" in Documentation/RCU/Design/Requirements/Requirements.rst and the section "GUARANTEES" in Documentation/memory-barriers.txt

Finn Thain (2):
documentation: Discourage alignment assumptions
atomic: Specify alignment for atomic_t and atomic64_t

Peter Zijlstra (1):
atomic: Add alignment check to instrumented atomic operations

 Documentation/core-api/unaligned-memory-access.rst |  7 -------
 include/asm-generic/atomic64.h                     |  2 +-
 include/linux/instrumented.h                       |  4 ++++
 include/linux/types.h                              |  2 +-
 lib/Kconfig.debug                                  | 10 ++++++++++
 5 files changed, 16 insertions(+), 9 deletions(-)

2.49.1

Discourage assumptions that simply don't hold for all Linux ABIs. Exceptions to the natural alignment rule for scalar types include long long on i386 and sh.

---
 Documentation/core-api/unaligned-memory-access.rst | 7 -------
 1 file changed, 7 deletions(-)

index 5ceeb80eb539..1390ce2b7291 100644
@@ -40,9 +40,6 @@ The rule mentioned above forms what we refer to as natural alignment:
 When accessing N bytes of memory, the base memory address must be evenly
 divisible by N, i.e. addr % N == 0.
 
-When writing code, assume the target architecture has natural alignment
-requirements.
-
 In reality, only a few architectures require natural alignment on all sizes
 of memory access. However, we must consider ALL supported architectures;
 writing code that satisfies natural alignment requirements is the easiest way

@@ -103,10 +100,6 @@ Therefore, for standard structure types you can always rely on the compiler
 to pad structures so that accesses to fields are suitably aligned (assuming
 you do not cast the field to a type of different length).
 
-Similarly, you can also rely on the compiler to align variables and function
-parameters to a naturally aligned scheme, based on the size of the type of
-the variable.
-
 At this point, it should be clear that accessing a single byte (u8 or char)
 will never cause an unaligned access, because all memory addresses are evenly
 divisible by one.

2.49.1

Some recent commits incorrectly assumed 4-byte alignment of locks. That assumption fails on Linux/m68k (and, interestingly, would have failed on Linux/cris also). Specify the minimum alignment of atomic variables for fewer surprises and (hopefully) better performance.

Consistent with i386, atomic64_t is not given natural alignment here.

---
Changed since v1:
- atomic64_t now gets an __aligned attribute too.
- The 'Fixes' tag has been dropped because Lance sent a different fix for commit e711faaafbe5 ("hung_task: replace blocker_mutex with encoded blocker") that's suitable for -stable.

---
 include/asm-generic/atomic64.h | 2 +-
 include/linux/types.h          | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

index 100d24b02e52..7ae82ac17645 100644
@@ -10,7 +10,7 @@
 #include <linux/types.h>
 
 typedef struct {
-	s64 counter;
+	s64 counter __aligned(sizeof(long));
 } atomic64_t;
 
 #define ATOMIC64_INIT(i)	{ (i) }

index 6dfdb8e8e4c3..cd5b2b0f4b02 100644
@@ -179,7 +179,7 @@ typedef phys_addr_t resource_size_t;
 typedef unsigned long irq_hw_number_t;
 
 typedef struct {
-	int counter;
+	int counter __aligned(sizeof(int));
 } atomic_t;
 
 #define ATOMIC_INIT(i) { (i) }

2.49.1

Hi Finn,

Thanks for your patch!

You forgot to drop this line.

Gr{oetje,eeting}s,

Geert

Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- [email protected]

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

Yeah, please use natural alignment for all atomic types.

Right, so x86 as a whole will accept unaligned data for its atomics, but at terrible overhead. Modern x86 grew (optional) trap on unaligned, and we've made sure the kernel is 'clean'.

With that, the i586 cmpxchg8b instruction very much wants 8 byte alignment. Similarly, the x86_64 cmpxchg16b instruction wants 16 bytes alignment.

Right, I forgot to check includes in arch/x86/include. (I had assumed this definition was relevant to that architecture, hence the sizeof(long), in order to stick to native alignment on x86-32.)

By "native alignment", do you mean "natural alignment" here?

Ok

Yes, that's what I meant.

Arnd

Hi Finn,

Thanks for your patch!

  553 |                         prefix ## suffix();
         \
      |                         ^~~~~~

      |         ^~~~~~~~~~~~~~~~~~~
include/linux/build_bug.h:39:37: note: in expansion of macro
‘compiletime_assert’
   39 | #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
      |                                     ^~~~~~~~~~~~~~~~~~
include/linux/build_bug.h:50:9: note: in expansion of macro ‘BUILD_BUG_ON_MSG’
   50 |         BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
      |         ^~~~~~~~~~~~~~~~
kernel/bpf/rqspinlock.c:695:9: note: in expansion of macro ‘BUILD_BUG_ON’
  695 |         BUILD_BUG_ON(__alignof__(rqspinlock_t) !=
__alignof__(struct bpf_res_spin_lock));
      |         ^~~~~~~~~~~~

Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- [email protected]

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

Yes, I noticed that also, after I started building with defconfigs. I pushed a new patch to my github repo.

https://github.com/fthain/linux/tree/atomic_t

Hi Finn,

Thanks, the updated version fixes the issue.

Gr{oetje,eeting}s,

Geert

Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- [email protected]

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

I had a look at that repository, and I think the extra annotations you added on a lot of structures have the same mistake that I made when looking at annotating ABI structures for -malign-int earlier:

Adding __aligned__((sizeof(long))) to a structure definition only changes the /outer/ alignment of the structure itself, so e.g.

struct {
    short a;
    int b;
} __attribute__((aligned(sizeof(long))));

creates a structure with 4-byte alignment and 8-byte size when building with -mno-align-int, but the padding is added after 'b', which is still misaligned.

In order to align members the same way that -malign-int does, each unaligned member needs a separate attribute to force aligning it, like

struct {
    short a;
    int b __attribute__((aligned(sizeof(int))));
};

This is different from how __attribute__((packed)) works, as that is always applied to each member of the struct if you add it to the end.

Arnd

Right. I should have dropped those changes earlier.

@@ -8,7 +8,7 @@ struct vfsmount;

 struct path {
        struct vfsmount *mnt;
        struct dentry *dentry;

-} __randomize_layout;
+} __aligned(sizeof(long)) __randomize_layout;

There's no need: struct path contains a struct dentry, which contains a seqcount_spinlock_t, which contains a spinlock_t which contains an atomic_t member, which is explicitly aligned.

Despite that, there's still some kmem cache or other allocator somewhere that has produced some misaligned path and dentry structures. So we get misaligned atomics somewhere in the VFS and TTY layers. I was unable to find those allocations.

Sure but -mno-align-int isn't particularly relevant here as it's only for m68k. Besides, I'd like to avoid the mess that would create.

The basic aim here is to naturally align both atomic_t and atomic64_t on all architectures (my testing is confined to m68k) and to try to find out what that would cost and what benefit it might bring.

The patches you're reviewing were a hack to try to silence the WARN from CONFIG_DEBUG_ATOMIC, because that way I could try to assess the cost/benefit.

Ok, I see. Those would certainly be good to find. I would assume that all kmem caches have a sensible alignment on each architecture, but I think the definition in linux/slab.h actually ends up setting the minimum to 2 here:

#ifndef ARCH_KMALLOC_MINALIGN
#define ARCH_KMALLOC_MINALIGN __alignof__(unsigned long long) #elif ARCH_KMALLOC_MINALIGN > 8
#define KMALLOC_MIN_SIZE ARCH_KMALLOC_MINALIGN #define KMALLOC_SHIFT_LOW ilog2(KMALLOC_MIN_SIZE) #endif

#ifndef ARCH_SLAB_MINALIGN
#define ARCH_SLAB_MINALIGN __alignof__(unsigned long long) #endif

Maybe we should just change __alignof__(unsigned long long) to a plain '8' here and make that the minimum alignment everywhere, same as the atomic64_t alignment change.

Alternatively, we can keep the __alignof__ here in order to reduce padding on architectures with a default 4-byte alignment for __u64, but then override ARCH_SLAB_MINALIGN and ARCH_KMALLOC_MINALIGN on m68k to be '4' instead of '2'.

Arnd

It would be better (less wasteful of memory) to specify the alignment parameter to kmem_cache_create() only at those call sites where it matters.

Raising that to 4 would probably have little or no effect (for m68k or any other arch). Back when I prepared the RFC patch series, I instrumented create_cache() in mm/slab_common.c, and those caches that were allocated at boot (for my usual minimal m68k .config) were already aligned to 4 bytes or 16.

Also, increasing ARCH_SLAB_MINALIGN to 8 didn't solve the VFS/TTY layer problem I have with CONFIG_DEBUG_ATOMIC on m68k. So the culprit is not the obvious suspect (a kmem cache of objects with atomic64_t members). There's some other allocator at work and it's aligning objects to 2 bytes not 4.

I think that's the idea: __alignof__(unsigned long long) is the alignment that the ABI requires for allocating arbitrary data structures that may contain 64-bit integers, so this is already the minimum. Architectures that may have noncoherent DMA master devices often need to raise the ARCH_KMALLOC_MINALIGN to the cache line size but leave the ARCH_SLAB_MINALIGN at the ABI-required minimum value. Any caller that needs a higher alignment than the ABI minimum needs to specify that.

Ok

In that case, my guess would be something that concatenates structures in some variant of

struct s1 {
     int a;
     short b;
     int rest[];
};

struct s2 {
     atomic_t a;
};

     struct s1 *obj1 = kmalloc(sizeof(struct s1) + sizeof(struct s2), GFP_KERNEL);
     struct s2 *obj2 = (void*)&obj1[1];

which causes problems because s2 has a larger alignment than s1. We've had problems with DMA to structures like this in the past.

The more common construct that does

struct s1 {
     short a;
     struct s2;
};

should not produce misaligned members in the inner struct, unless the outer one has a __packed attribute. Unfortunately we disabled -Wpacked-not-aligned, which would otherwise catch that problem.

Arnd

It turned out that the problem wasn't dynamic allocations, it was a local variable in the core locking code (kernel/locking/rwsem.c): a misaligned long used with an atomic operation (cmpxchg). To get natural alignment for 64-bit quantities, I had to align other local variables as well, such as the one in ktime_get_real_ts64_mg() that's used with atomic64_try_cmpxchg(). The atomic_t branch in my github repo has the patches I wrote for that.

To silence the misalignment WARN from CONFIG_DEBUG_ATOMIC, for 64-bit atomic operations, for my small m68k .config, it was also necesary to increase ARCH_SLAB_MINALIGN to 8. However, I'm not advocating a ARCH_SLAB_MINALIGN increase, as that wastes memory. I think it might be more useful to limit the alignment test for CONFIG_DEBUG_ATOMIC, as follows.

index 402a999a0d6b..cd569a87c0a8 100644
@@ -68,7 +68,7 @@ static __always_inline void instrument_atomic_read(const volatile void *v, size_
 {
 	kasan_check_read(v, size);
 	kcsan_check_atomic_read(v, size);
-	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (size - 1)));
+	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (size - 1) & 3));
 }
 
 /**

@@ -83,7 +83,7 @@ static __always_inline void instrument_atomic_write(const volatile void *v, size
 {
 	kasan_check_write(v, size);
 	kcsan_check_atomic_write(v, size);
-	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (size - 1)));
+	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (size - 1) & 3));
 }
 
 /**

@@ -98,7 +98,7 @@ static __always_inline void instrument_atomic_read_write(const volatile void *v,
 {
 	kasan_check_write(v, size);
 	kcsan_check_atomic_read_write(v, size);
-	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (size - 1)));
+	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (size - 1) & 3));
 }
 
 /**

It looks like the variable you get the warning for is not even the atomic64_t but the 'old' argument to atomic64_try_cmpxchg(), at least in some of the cases you found if not all of them.

I don't see where why there is a requirement to have that aligned at all, even if we do require the atomic64_t to be naturally aligned, and I would expect the same warning to hit on x86-32 and the other architectures with 4-byte alignment of u64 variable on stack and .data.

Have you tried to quantify the memory waste here? I assume that most slab allocations are already 8-byte aligned, at least kmalloc() with size>4, while custom caches are usually done for larger structures where an extra average of 2 bytes per allocation may not be that bad.

What is the alignment of stack variables on m68k? E.g. if you have a function with two local variables, would that still be able to trigger the check?

int f(atomic64_t *a)

{
     u16 pad;
     u64 old;
     
     g(&pad);
     atomic64_try_cmpxchg(a, &old, 0);
}

Since there is nothing telling the compiler that the 'old' argument to atomic*_try_cmpcxchg() needs to be naturally aligned, maybe that check should be changed to only test for the ABI-guaranteed alignment? I think that would still be needed on x86-32.

Arnd

index 9409a6ddf3e0..e57763a889bd 100644
@@ -1276,7 +1276,7 @@ atomic_try_cmpxchg(atomic_t *v, int *old, int new)
 {
 	kcsan_mb();
 	instrument_atomic_read_write(v, sizeof(*v));
-	instrument_atomic_read_write(old, sizeof(*old));
+	instrument_atomic_read_write(old, alignof(*old));
 	return raw_atomic_try_cmpxchg(v, old, new);
 }
 

@@ -1298,7 +1298,7 @@ static __always_inline bool
 atomic_try_cmpxchg_acquire(atomic_t *v, int *old, int new)
 {
 	instrument_atomic_read_write(v, sizeof(*v));
-	instrument_atomic_read_write(old, sizeof(*old));
+	instrument_atomic_read_write(old, alignof(*old));
 	return raw_atomic_try_cmpxchg_acquire(v, old, new);
 }
 

@@ -1321,7 +1321,7 @@ atomic_try_cmpxchg_release(atomic_t *v, int *old, int new)
 {
 	kcsan_release();
 	instrument_atomic_read_write(v, sizeof(*v));
-	instrument_atomic_read_write(old, sizeof(*old));
+	instrument_atomic_read_write(old, alignof(*old));
 	return raw_atomic_try_cmpxchg_release(v, old, new);
 }
 

@@ -1343,7 +1343,7 @@ static __always_inline bool
 atomic_try_cmpxchg_relaxed(atomic_t *v, int *old, int new)
 {
 	instrument_atomic_read_write(v, sizeof(*v));
-	instrument_atomic_read_write(old, sizeof(*old));
+	instrument_atomic_read_write(old, alignof(*old));
 	return raw_atomic_try_cmpxchg_relaxed(v, old, new);
 }
 

@@ -2854,7 +2854,7 @@ atomic64_try_cmpxchg(atomic64_t *v, s64 *old, s64 new)
 {
 	kcsan_mb();
 	instrument_atomic_read_write(v, sizeof(*v));
-	instrument_atomic_read_write(old, sizeof(*old));
+	instrument_atomic_read_write(old, alignof(*old));
 	return raw_atomic64_try_cmpxchg(v, old, new);
 }
 

@@ -2876,7 +2876,7 @@ static __always_inline bool
 atomic64_try_cmpxchg_acquire(atomic64_t *v, s64 *old, s64 new)
 {
 	instrument_atomic_read_write(v, sizeof(*v));
-	instrument_atomic_read_write(old, sizeof(*old));
+	instrument_atomic_read_write(old, alignof(*old));
 	return raw_atomic64_try_cmpxchg_acquire(v, old, new);
 }
 

@@ -2899,7 +2899,7 @@ atomic64_try_cmpxchg_release(atomic64_t *v, s64 *old, s64 new)
 {
 	kcsan_release();
 	instrument_atomic_read_write(v, sizeof(*v));
-	instrument_atomic_read_write(old, sizeof(*old));
+	instrument_atomic_read_write(old, alignof(*old));
 	return raw_atomic64_try_cmpxchg_release(v, old, new);
 }
 

@@ -2921,7 +2921,7 @@ static __always_inline bool
 atomic64_try_cmpxchg_relaxed(atomic64_t *v, s64 *old, s64 new)
 {
 	instrument_atomic_read_write(v, sizeof(*v));
-	instrument_atomic_read_write(old, sizeof(*old));
+	instrument_atomic_read_write(old, alignof(*old));
 	return raw_atomic64_try_cmpxchg_relaxed(v, old, new);
 }
 

@@ -4432,7 +4432,7 @@ atomic_long_try_cmpxchg(atomic_long_t *v, long *old, long new)
 {
 	kcsan_mb();
 	instrument_atomic_read_write(v, sizeof(*v));
-	instrument_atomic_read_write(old, sizeof(*old));
+	instrument_atomic_read_write(old, alignof(*old));
 	return raw_atomic_long_try_cmpxchg(v, old, new);
 }
 

@@ -4454,7 +4454,7 @@ static __always_inline bool
 atomic_long_try_cmpxchg_acquire(atomic_long_t *v, long *old, long new)
 {
 	instrument_atomic_read_write(v, sizeof(*v));
-	instrument_atomic_read_write(old, sizeof(*old));
+	instrument_atomic_read_write(old, alignof(*old));
 	return raw_atomic_long_try_cmpxchg_acquire(v, old, new);
 }
 

@@ -4477,7 +4477,7 @@ atomic_long_try_cmpxchg_release(atomic_long_t *v, long *old, long new)
 {
 	kcsan_release();
 	instrument_atomic_read_write(v, sizeof(*v));
-	instrument_atomic_read_write(old, sizeof(*old));
+	instrument_atomic_read_write(old, alignof(*old));
 	return raw_atomic_long_try_cmpxchg_release(v, old, new);
 }
 

@@ -4499,7 +4499,7 @@ static __always_inline bool
 atomic_long_try_cmpxchg_relaxed(atomic_long_t *v, long *old, long new)
 {
 	instrument_atomic_read_write(v, sizeof(*v));
-	instrument_atomic_read_write(old, sizeof(*old));
+	instrument_atomic_read_write(old, alignof(*old));
 	return raw_atomic_long_try_cmpxchg_relaxed(v, old, new);
 }
 

Right -- there's only one memory operand in a CAS instruction on m68k, and there's only one pointer in the C implementation in asm-generic.

I think it's entirely workload dependent. The memory efficiency question comes down to the misalignment distance as a proportion of the size of the allocation.

I assume so:

int foo(void) {
    short s;
    long long ll;
    return alignof(ll);
}

# Compilation provided by Compiler Explorer at https://godbolt.org/
foo():

        link.w %fp,#0
        moveq #2,%d0
        unlk %fp
        rts

I don't know why we would check the alignment of the 'old' quantity. It's going to be loaded into a register before being used, right?

This just returns the guaranteed alignment of the 'long long' type based on -malign-int/-mno-align-int. Checking again I find that gcc's m68k-linux target aligns stack allocations to 4 bytes, though the m68k-unknown target apparently keeps the 2 byte alignment:

https://gcc.gnu.org/legacy-ml/gcc-patches/2007-09/msg01572.html

https://godbolt.org/z/48fGMj56W

Surprisingly the godbolt.org link also shows a significant overhead when building the same code with -malign-int in the second tab. This is unrelated to the issue here, but I wonder if that is something to report to the gcc bug tracker if we ever get to building the kernel with -malign-int. Similarly, I noticed that clang does not support the -malign-int flag on m68k at all.

I was wondering about that as well, but checking for alignof(*old) probably can't hurt. The only architectures that actually have a custom arch_try_cmpxchg*() are s390 and x86 and those don't care about alignmnent of 'old', but it's possible that another architecture that can't handle unaligned load/store would add an inline asm implementation in the future and break if an alignment fixup happens in the middle of an ll/sc loop.

Arnd

That hypothetical future requirement seems improbable to me. Moreover, would such an architecture have a need for CONFIG_DEBUG_ATOMIC?

Right, we still need the effect of instrument_read_write() for kasan/kcsan sanitizing with the correct size, only the alignment check from the instrument_atomic_read_write() is wrong here here.

It looks like Mark Rutland fixed the try_cmpxchg() function this way in commit ec570320b09f ("locking/atomic: Correct (cmp)xchg() instrumentation"), but for some reason we still have the wrong annotation on the atomic_try_cmpxchg() helpers.

index 592f3ec89b5f..9c1d53f81eb2 100755
@@ -12,7 +12,7 @@ gen_param_check()
 	local arg="$1"; shift
 	local type="${arg%%:*}"
 	local name="$(gen_param_name "${arg}")"
-	local rw="write"
+	local rw="atomic_write"
 
 	case "${type#c}" in
 	i) return;;

@@ -20,14 +20,17 @@ gen_param_check()
 
 	if [ ${type#c} != ${type} ]; then
 		# We don't write to constant parameters.
-		rw="read"
+		rw="atomic_read"
+	elif [ "${type}" = "p" ] ; then
+		# The "old" argument in try_cmpxchg() gets accessed non-atomically
+		rw="read_write"
 	elif [ "${meta}" != "s" ]; then
 		# An atomic RMW: if this parameter is not a constant, and this atomic is
 		# not just a 's'tore, this parameter is both read from and written to.
-		rw="read_write"
+		rw="atomic_read_write"
 	fi
 
-	printf "\tinstrument_atomic_${rw}(${name}, sizeof(*${name}));\n"
+	printf "\tinstrument_${rw}(${name}, sizeof(*${name}));\n"
 }
 
 #gen_params_checks(meta, arg...)

I don't see anything wrong with this. The result looks good.

Thanks for having a look, I've sent it out as a proper patch now.

Arnd

Hi Finn,

So cmpxchg() and friends should not take a volatile void *, but (new) properly-aligned types, using the new _Generic()?

Probably ARCH_SLAB_MINALIGN should be 4 on m68k. Somehow I thought that was already the case, but it is __alignof__(unsigned long long) = 2.

Did you check what would be the actual impact of increasing it to 4 or 8?

Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- [email protected]

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

I agree -- setting ARCH_SLAB_MINALIGN to 4 would be better style, and may avoid surprises in future. Right now that won't have any effect because that value gets increased to sizeof(void *) by calculate_alignment() and gets increased to ARCH_KMALLOC_MINALIGN or ARCH_DMA_MINALIGN by __kmalloc_minalign().

How about (sizeof(atomic_long_t) - 1)?

Can you comment on this, Peter?

Hi Finn,

Ah, so there it happens...

Thanks for checking!

Gr{oetje,eeting}s,

Geert

Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- [email protected]

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

From: Peter Zijlstra <[email protected]>

Add a Kconfig option for debug builds which logs a warning when an instrumented atomic operation takes place at some location that isn't a long word boundary. Some platforms don't trap for this.

---
This patch differs slightly from Peter's code which checked for natural alignment.

---
 include/linux/instrumented.h |  4 ++++
 lib/Kconfig.debug            | 10 ++++++++++
 2 files changed, 14 insertions(+)

index 711a1f0d1a73..55f5685971a1 100644
@@ -7,6 +7,7 @@
 #ifndef _LINUX_INSTRUMENTED_H
 #define _LINUX_INSTRUMENTED_H
 
+#include <linux/bug.h>
 #include <linux/compiler.h>
 #include <linux/kasan-checks.h>
 #include <linux/kcsan-checks.h>

@@ -67,6 +68,7 @@ static __always_inline void instrument_atomic_read(const volatile void *v, size_
 {
 	kasan_check_read(v, size);
 	kcsan_check_atomic_read(v, size);
+	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (sizeof(long) - 1)));
 }
 
 /**

@@ -81,6 +83,7 @@ static __always_inline void instrument_atomic_write(const volatile void *v, size
 {
 	kasan_check_write(v, size);
 	kcsan_check_atomic_write(v, size);
+	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (sizeof(long) - 1)));
 }
 
 /**

@@ -95,6 +98,7 @@ static __always_inline void instrument_atomic_read_write(const volatile void *v,
 {
 	kasan_check_write(v, size);
 	kcsan_check_atomic_read_write(v, size);
+	WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ATOMIC) && ((unsigned long)v & (sizeof(long) - 1)));
 }
 
 /**

index ebe33181b6e6..d82626b7d6be 100644
@@ -1363,6 +1363,16 @@ config DEBUG_PREEMPT
 	  depending on workload as it triggers debugging routines for each
 	  this_cpu operation. It should only be used for debugging purposes.
 
+config DEBUG_ATOMIC
+	bool "Debug atomic variables"
+	depends on DEBUG_KERNEL
+	help
+	  If you say Y here then the kernel will add a runtime alignment check
+	  to atomic accesses. Useful for architectures that do not have trap on
+	  mis-aligned access.
+
+	  This option has potentially significant overhead.
+
 menu "Lock Debugging (spinlocks, mutexes, etc...)"
 
 config LOCK_DEBUGGING_SUPPORT

2.49.1

It wasn't supposed to mandate a larger alignment in practice. I considered doing something like (unsigned long)v & (size - 1) & (sizeof(long) - 1) but decided that the extra overhead probably wouldn't be worthwhile, if in practice, no-one is doing atomic ops on shorts or chars. I will revisit this.

When you do atomic operations on atomic_t or atomic64_t, (sizeof(long)
- 1) probably doesn't make much sense. But atomic operations get used on scalar types (aside from atomic_t and atomic64_t) that don't have natural alignment. Please refer to the other thread about this:
https://lore.kernel.org/all/[email protected]/

atomic_t is aligned at 4 bytes, you're now mandating it is aligned at 8 bytes (on LP64), this cannot be right.

kernel/locking/qspinlock.c:xchg_tail() does xchg_relaxed(&lock->tail, ...) which is u16. Again, you cannot mandate 8 bytes here.

Perhaps set ARCH_SLAB_MINALIGN ?

OK. I will change it back to your code (i.e. mandate natural alignment).

That's not going to help much. The 850 byte offset of task_works into struct task_struct and the 418 byte offset of exit_state in struct task_struct are already misaligned.

But that's all moot, if you intended that CONFIG_DEBUG_ATOMIC should complain about any deviation from natural alignment. I still don't have any performance measurements but I'm willing to assume there's a penalty for such deviation.

Has there been any progress on building m68k kernels with -mint-align? IIRC there are only a small number of uapi structures that need __packed annotations to maintain the existing syscall ABI.

Arnd

Not that I know of.

Packing uapi structures (and adopting -malign-int) sounds easier than the alternative, which might be to align certain internal kernel struct members, on a case-by-case basis, where doing so could be shown to improve performance on some architecture or other (while keeping -mno-align-int).

Well, it's easy to find all the structs that belong to the uapi, but it's not easy to find all the internal kernel structs that describe MMIO registers. For -malign-int, both kinds of structs are a problem.

If better performance is to be had, my guess is that aligning atomic_t will get 80% of it (just an appeal to the Pareto principle, FWIW...)

Hi Finn,

CC Adrian,

indeed.

For structures under arch/m68k/include/asm/, just create a single C file that calculates sizeof() of each structure, and compare the generated code with and without -malign-int. Any differences should be investigated, and attributed when needed.

For structures inside m68k-specific drivers, do something similar inside those drivers ('git grep "struct\s*[a-zA-Z0-9_]*\s*{"' is your friend).

Most Amiga-specific drivers should be fine, as they were used on APUS (PowerPC) before. I guess the same is true for some of the Mac-specific drivers that are shared with PowerPC.

Gr{oetje,eeting}s,

Geert

Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- [email protected]

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

Well, it "sounds easier". But I doubt that it really is easier.

There's something to be said for adding static_assert() checks for the structs that belong to all fixed interfaces. The patches to actually pack of struct members could take place after that.

All of which is a lot of work, compared to specifying alignment for those core kernel data structures where doing so improves performance. This approach is platform neutral; it's not just m68k that benefits.

Right, especially since those structure definitions are more likely to be used on older drivers, there are probably some that do happen on m68k. On the other hand, any driver that is portable to non-m68k targets won't have this problem.

I tried a trivial m68k defconfig build with "-Wpadded -malign-int" and found 3021 instances of structure padding, compared to 2271 without -malign-int. That is still something one could very reasonably go through with 'vim -q output', as almost all of them are obviously kernel-internal structures and not problematic.

A quick manual scan only shows a number of uapi structures but very few drivers. There are a few hardware structures that are internally packed but in a structure that has gets an extra two bytes of harmless padding at the end (struct CUSTOM, struct amiga_hw_present, struct atari_hw_present, struct TT_DMA, struct ppc_regs). The only ones I could find that actually seemed suspicious are:

arch/m68k/include/asm/atarihw.h:426:10: warning: padding struct to align 'dst_address' [-Wpadded] arch/m68k/include/asm/openprom.h:76:13: warning: padding struct to align 'boot_dev_ctrl' [-Wpadded] drivers/net/ethernet/i825xx/82596.c:250:1: warning: padding struct size to alignment boundary

I'm sure there are a few more, but probably not a lot more. See https://pastebin.com/Z2bjnD0G for the full list. I've also tried annotating the ones that show up in defconfig, see diffstat below.

Unfortunately I found no way of annotating a struct a whole to use the traditional padding rules: "#pragma pack(push, 2)" lowers the alignment of all struct members to two bytes, including those with an explicit alignment like __aligned_u64. A struct-wide __attribute__((packed, aligned(2))) seems even worse, as it makes all members inside of the struck tightly packed and only aligns the struct itself.

This means all misaligned members have to be individually marked as __packed, unless someone comes up with another type of annotation.

arch/m68k selects CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS for anything other than Dragonball, so I would not expect much performance difference at all, unless CASL on unaligned data ends up causing alignment traps as it does on most architectures.

Arnd

 arch/m68k/atari/atakeyb.c                          |  2 +-
 arch/m68k/include/asm/amigahw.h                    |  4 +--
 arch/m68k/include/asm/atarihw.h                    |  6 ++--
 arch/m68k/include/asm/mvme147hw.h                  |  2 +-
 arch/m68k/include/asm/openprom.h                   |  2 +-
 arch/m68k/include/uapi/asm/ptrace.h                |  2 +-
 arch/m68k/include/uapi/asm/sigcontext.h            |  2 +-
 arch/m68k/include/uapi/asm/stat.h                  |  4 +--
 drivers/net/ethernet/i825xx/82596.c                |  4 +--
 include/uapi/asm-generic/termios.h                 |  2 +-
 include/uapi/linux/acct.h                          |  2 +-
 include/uapi/linux/ax25.h                          |  6 ++--
 include/uapi/linux/blktrace_api.h                  |  2 +-
 include/uapi/linux/btrfs.h                         |  2 +-
 include/uapi/linux/cdrom.h                         | 36 +++++++++++-----------
 include/uapi/linux/dlm.h                           |  2 +-
 include/uapi/linux/dqblk_xfs.h                     |  2 +-
 include/uapi/linux/ethtool.h                       | 14 ++++-----
 include/uapi/linux/fb.h                            |  6 ++--
 include/uapi/linux/fd.h                            |  6 ++--
 include/uapi/linux/filter.h                        |  2 +-
 include/uapi/linux/hdlc/ioctl.h                    |  6 ++--
 include/uapi/linux/hiddev.h                        |  2 +-
 include/uapi/linux/i2c-dev.h                       |  2 +-
 include/uapi/linux/i2c.h                           |  2 +-
 include/uapi/linux/if.h                            |  2 +-
 include/uapi/linux/if_arcnet.h                     |  4 +--
 include/uapi/linux/if_bonding.h                    |  2 +-
 include/uapi/linux/if_bridge.h                     | 14 ++++-----
 include/uapi/linux/if_link.h                       |  2 +-
 include/uapi/linux/if_plip.h                       |  2 +-
 include/uapi/linux/if_pppox.h                      |  2 +-
 include/uapi/linux/if_vlan.h                       |  2 +-
 include/uapi/linux/inet_diag.h                     |  2 +-
 include/uapi/linux/input.h                         |  4 +--
 include/uapi/linux/ip6_tunnel.h                    |  4 +--
 include/uapi/linux/kd.h                            |  2 +-
 include/uapi/linux/llc.h                           |  2 +-
 include/uapi/linux/loop.h                          |  2 +-
 include/uapi/linux/mctp.h                          |  4 +--
 include/uapi/linux/mptcp.h                         |  2 +-
 include/uapi/linux/mroute.h                        |  4 +--
 include/uapi/linux/mroute6.h                       |  6 ++--
 include/uapi/linux/msdos_fs.h                      |  2 +-
 include/uapi/linux/msg.h                           |  6 ++--
 include/uapi/linux/mtio.h                          |  2 +-
 include/uapi/linux/netfilter/ipset/ip_set.h        |  4 +--
 include/uapi/linux/netfilter/nf_nat.h              |  2 +-
 include/uapi/linux/netfilter/x_tables.h            |  8 ++---
 include/uapi/linux/netfilter/xt_HMARK.h            |  2 +-
 include/uapi/linux/netfilter/xt_TPROXY.h           |  4 +--
 include/uapi/linux/netfilter/xt_connbytes.h        |  2 +-
 include/uapi/linux/netfilter/xt_connmark.h         |  6 ++--
 include/uapi/linux/netfilter/xt_conntrack.h        |  4 +--
 include/uapi/linux/netfilter/xt_esp.h              |  2 +-
 include/uapi/linux/netfilter/xt_hashlimit.h        |  6 ++--
 include/uapi/linux/netfilter/xt_helper.h           |  2 +-
 include/uapi/linux/netfilter/xt_ipcomp.h           |  2 +-
 include/uapi/linux/netfilter/xt_iprange.h          |  2 +-
 include/uapi/linux/netfilter/xt_l2tp.h             |  2 +-
 include/uapi/linux/netfilter/xt_mac.h              |  2 +-
 include/uapi/linux/netfilter/xt_mark.h             |  2 +-
 include/uapi/linux/netfilter/xt_owner.h            |  2 +-
 include/uapi/linux/netfilter/xt_realm.h            |  2 +-
 include/uapi/linux/netfilter/xt_recent.h           |  4 +--
 include/uapi/linux/netfilter/xt_set.h              |  4 +--
 include/uapi/linux/netfilter/xt_statistic.h        |  2 +-
 include/uapi/linux/netfilter/xt_tcpmss.h           |  2 +-
 include/uapi/linux/netfilter/xt_tcpudp.h           |  2 +-
 include/uapi/linux/netfilter/xt_time.h             |  2 +-
 include/uapi/linux/netfilter/xt_u32.h              |  6 ++--
 include/uapi/linux/netfilter_arp/arp_tables.h      |  2 +-
 include/uapi/linux/netfilter_arp/arpt_mangle.h     |  2 +-
 include/uapi/linux/netfilter_bridge/ebt_802_3.h    |  2 +-
 include/uapi/linux/netfilter_bridge/ebt_arp.h      |  2 +-
 include/uapi/linux/netfilter_bridge/ebt_arpreply.h |  2 +-
 include/uapi/linux/netfilter_bridge/ebt_mark_m.h   |  2 +-
 include/uapi/linux/netfilter_bridge/ebt_nat.h      |  2 +-
 include/uapi/linux/netfilter_bridge/ebt_stp.h      |  4 +--
 include/uapi/linux/netfilter_bridge/ebt_vlan.h     |  2 +-
 include/uapi/linux/netfilter_ipv4/ipt_ah.h         |  2 +-
 include/uapi/linux/netfilter_ipv6/ip6_tables.h     |  2 +-
 include/uapi/linux/netfilter_ipv6/ip6t_ah.h        |  2 +-
 include/uapi/linux/netfilter_ipv6/ip6t_frag.h      |  2 +-
 include/uapi/linux/netfilter_ipv6/ip6t_opts.h      |  2 +-
 include/uapi/linux/netfilter_ipv6/ip6t_rt.h        |  2 +-
 include/uapi/linux/netfilter_ipv6/ip6t_srh.h       |  2 +-
 include/uapi/linux/netrom.h                        |  2 +-
 include/uapi/linux/nfs_idmap.h                     |  2 +-
 include/uapi/linux/nfs_mount.h                     |  2 +-
 include/uapi/linux/pkt_cls.h                       |  2 +-
 include/uapi/linux/rds.h                           |  4 +--
 include/uapi/linux/rose.h                          |  8 ++---
 include/uapi/linux/route.h                         |  2 +-
 include/uapi/linux/rtc.h                           |  2 +-
 include/uapi/linux/sctp.h                          | 18 +++++------
 include/uapi/linux/sed-opal.h                      |  2 +-
 include/uapi/linux/sem.h                           |  2 +-
 include/uapi/linux/serial.h                        |  4 +--
 include/uapi/linux/soundcard.h                     |  8 ++---
 include/uapi/linux/taskstats.h                     |  2 +-
 include/uapi/linux/virtio_net.h                    |  4 +--
 include/uapi/linux/wireless.h                      |  4 +--
 include/uapi/linux/xfrm.h                          | 26 ++++++++--------
 include/uapi/mtd/mtd-abi.h                         |  2 +-
 include/uapi/rdma/hfi/hfi1_ioctl.h                 |  2 +-
 include/uapi/rdma/ib_user_ioctl_verbs.h            |  2 +-
 include/uapi/rdma/ib_user_mad.h                    |  2 +-
 109 files changed, 205 insertions(+), 204 deletions(-)

I believe it depends on the exact CPU. The 68060 user manual has a section called "Emulating CAS2 and CAS Misaligned" discussing the handling of such instances through software emulation. The 68040 user manual doesn't have any similar section. I haven't looked at the exception handlers in the kernel to see if such cases are being handled. The documentation says it results in an unimplemented integer exception, and the handler has to manually lock the bus while performing normal memory operations.

Brad Boyer
[email protected]

The 68040 still handles them in hardware.

Andreas Schwab, [email protected]
GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510  2552 DF73 E780 A9DA AEC1
"And now for something completely different."

Yes, but only inasmuchas drivers are completely portable. Any data structure accessed or declared using #if conditional code would defeat that kind of reasoning.

So, 3021 is the number of structs potentially needing to be checked? This is not the kind of upper bound I consider feasible for careful manual editing (it's worse than I thought). We need the compiler to help.

I wouldn't assume that padding at the end is inconsequential. I think the analysis requires comparing object code. E.g. add the same padding explicitly to see whether it alters object code under -mno-align-int.

Right. The trick will be to annotate the affected struct members by automatic program transformation.

I think the answer to the performance question would depend on both choice of workload and choice of architecture. I haven't done any measurements on this patch series yet.

Thanks for sending these results. I was looking into doing something similar using pahole but found that difficult. When I #include'd all the headers of interest, the thing doesn't build. I abandoned that approach and concluded that the way forward was to get the compiler to do the analysis (perhaps with a plug-in), during a normal kernel build, rather than during compilation of some contrived mess containing all the headers and all the structs.

It would be sufficient to have a plug-in to list the sites of all members potentially needing annnotation. The list could be manually pared down and the actual annotating could be scripted. Note that this list would be shorter than a -Wpadded list, because there's a bunch of padding that's not relevant.

Sure, but those are extremely rare. If there is a structure definition, it's usually done in a way that describes the hardware and is already densely packed. There are only a handful of compile-time checks for CONFIG_M68K and __m68k__ in the kernel, and none of those are around data structure definitions.

It's the number of add pads, so there are some structures that show up many times in that list.

There would be significantly more than those 3021 if you include drivers that are not in defconfig. An 'allmodconfig' build brings the number up to 17000, but it's easy to quickly bring that number down again by excluding stuff that is obviously specific to a random Arm SoC. I would also exclude all of fs/* and most of net/* as being clearly irrelevant here.

In a given file, there are likely many instances either in the same struct or a set of related structs, where all can be discarded after looking at the pattern in that file.
See the end of this email for a count by directory.

I think just doing the uapi headers is probably enough to avoid accidentally introducing a new ABI when either the kernel or userspace gets built with -malign-int, everything inside of the kernel is at most a bug that can be fixed later.

There is clearly a difference in uapi headers, especially when it gets passed through an ioctl that encodes the size of the structure in the command code.

For the structures I listed above, I can see no way it would actually make a difference.

I don't think automated transformation is going to work well here, as you may not want the same approach in each case, depending on what the code is:

- it may be enough to annotate a single member as packed in order to make the entire structure compatible
- some structures may have lots of misaligned members, but no holes, so a global __attribute__((packed, aligned(2))) on that struct is cleaner
- if there are holes, some strategic additions of explicit padding can be cleaner than annotating each misaligned member.
- automation won't be able to tell whether a structure is ABI relevant or not
- similarly, many files are not going to be interesting for m68k. E.g. if drivers/infiniband has an ABI that is different for -malign-int, that can likely be ignored because nobody cares in practice.

Yes, that should work to rule out many instances and to find out exactly where packing attributes are required.

    Arnd

      2 arch/m68k/atari
      5 arch/m68k/include/asm
     39 block
      1 block/fs
     11 block/partitions
     34 crypto
      6 crypto/asymmetric_keys
     10 drivers/accessibility/speakup
     11 drivers/android
     12 drivers/android/tests
     15 drivers/ata
      8 drivers/auxdisplay
     11 drivers/base
      5 drivers/base/firmware_loader
      2 drivers/base/firmware_loader/builtin
     10 drivers/base/regmap
      2 drivers/base/test
      2 drivers/bcma
     18 drivers/block
      2 drivers/block/aoe
      7 drivers/block/drbd
      4 drivers/block/null_blk
      7 drivers/block/rnbd
      2 drivers/block/zram
     55 drivers/bluetooth
      4 drivers/bus
      2 drivers/bus/mhi/ep
      4 drivers/bus/mhi/host
      3 drivers/cdx
      6 drivers/cdx/controller
      3 drivers/char
      8 drivers/char/hw_random
     35 drivers/char/ipmi
     11 drivers/char/tpm
      1 drivers/char/tpm/eventlog
      3 drivers/char/tpm/st33zp24
      1 drivers/char/xillybus
     48 drivers/clk
      5 drivers/clk/actions
      8 drivers/clk/bcm
      8 drivers/clk/imx
      6 drivers/clk/ingenic
     11 drivers/clk/mediatek
     24 drivers/clk/qcom
      2 drivers/clk/ralink
     21 drivers/clk/renesas
      6 drivers/clk/samsung
      1 drivers/clk/sifive
      9 drivers/clk/socfpga
     11 drivers/clk/sophgo
      3 drivers/clk/sprd
      2 drivers/clk/starfive
     17 drivers/clk/sunxi-ng
      1 drivers/clk/ti
      1 drivers/clk/versatile
      6 drivers/clk/visconti
      2 drivers/clk/xilinx
     43 drivers/clocksource
      1 drivers/comedi
     48 drivers/comedi/drivers
      6 drivers/comedi/drivers/tests
      9 drivers/counter
     37 drivers/crypto
      3 drivers/crypto/amlogic
      4 drivers/crypto/aspeed
     28 drivers/crypto/caam
     17 drivers/crypto/ccree
      3 drivers/crypto/hisilicon/sec
      7 drivers/crypto/inside-secure
      5 drivers/crypto/inside-secure/eip93
      2 drivers/crypto/intel/ixp4xx
      4 drivers/crypto/intel/keembay
      3 drivers/crypto/marvell/cesa
      7 drivers/crypto/qce
      4 drivers/crypto/starfive
      6 drivers/crypto/tegra
      7 drivers/crypto/virtio
      2 drivers/crypto/xilinx
      4 drivers/dax
      2 drivers/devfreq
      1 drivers/devfreq/event
     48 drivers/dma
      3 drivers/dma-buf
      1 drivers/dma-buf/heaps
      7 drivers/dma/amd/qdma
      4 drivers/dma/dw
      3 drivers/dma/dw-axi-dmac
      2 drivers/dma/lgm
      3 drivers/dma/mediatek
      5 drivers/dma/qcom
      1 drivers/dma/sf-pdma
      4 drivers/dma/sh
     12 drivers/dma/stm32
     14 drivers/dma/ti
     27 drivers/dma/xilinx
      5 drivers/dpll/zl3073x
     14 drivers/extcon
     11 drivers/firewire
      8 drivers/firmware
     35 drivers/firmware/arm_scmi
     10 drivers/firmware/arm_scmi/transports
      6 drivers/firmware/arm_scmi/vendors/imx
      1 drivers/firmware/cirrus
      2 drivers/firmware/google
      9 drivers/firmware/imx
      1 drivers/firmware/microchip
      2 drivers/firmware/samsung
     11 drivers/fpga
     10 drivers/fsi
      2 drivers/gnss
     47 drivers/gpio
      8 drivers/gpio/pinctrl
      4 drivers/gpu/drm
     12 drivers/gpu/drm/arm
     15 drivers/gpu/drm/arm/display/komeda
      1 drivers/gpu/drm/arm/display/komeda/d71
      2 drivers/gpu/drm/atmel-hlcdc
     40 drivers/gpu/drm/bridge
      9 drivers/gpu/drm/bridge/adv7511
      8 drivers/gpu/drm/bridge/analogix
      9 drivers/gpu/drm/bridge/cadence
      4 drivers/gpu/drm/bridge/imx
     11 drivers/gpu/drm/bridge/synopsys
      1 drivers/gpu/drm/clients
      4 drivers/gpu/drm/display
      5 drivers/gpu/drm/etnaviv
     15 drivers/gpu/drm/exynos
      3 drivers/gpu/drm/gud
      3 drivers/gpu/drm/hisilicon/kirin
      1 drivers/gpu/drm/imx/dc
     11 drivers/gpu/drm/imx/dcss
      2 drivers/gpu/drm/imx/ipuv3
      6 drivers/gpu/drm/ingenic
      9 drivers/gpu/drm/kmb
      4 drivers/gpu/drm/lima
      7 drivers/gpu/drm/logicvc
      2 drivers/gpu/drm/mcde
     39 drivers/gpu/drm/mediatek
     17 drivers/gpu/drm/meson
      2 drivers/gpu/drm/mxsfb
      8 drivers/gpu/drm/omapdrm
     41 drivers/gpu/drm/omapdrm/dss
     41 drivers/gpu/drm/panel
      2 drivers/gpu/drm/pl111
      8 drivers/gpu/drm/renesas/rcar-du
      2 drivers/gpu/drm/renesas/rz-du
     22 drivers/gpu/drm/rockchip
      1 drivers/gpu/drm/scheduler/tests
      4 drivers/gpu/drm/sitronix
      2 drivers/gpu/drm/solomon
      5 drivers/gpu/drm/sprd
     11 drivers/gpu/drm/sti
      3 drivers/gpu/drm/stm
     13 drivers/gpu/drm/sun4i
      1 drivers/gpu/drm/sysfb
     23 drivers/gpu/drm/tegra
     13 drivers/gpu/drm/tests
      1 drivers/gpu/drm/tests/sysfb
     10 drivers/gpu/drm/tidss
      3 drivers/gpu/drm/tiny
      2 drivers/gpu/drm/ttm
      4 drivers/gpu/drm/ttm/tests
      3 drivers/gpu/drm/v3d
      8 drivers/gpu/drm/virtio
      3 drivers/gpu/drm/vkms
      4 drivers/gpu/drm/vkms/tests
      9 drivers/gpu/drm/xlnx
     13 drivers/gpu/host1x
     11 drivers/gpu/host1x/hw
     13 drivers/gpu/ipu-v3
     10 drivers/greybus
    135 drivers/hid
      4 drivers/hid/i2c-hid
      5 drivers/hid/usbhid
      1 drivers/hte
    271 drivers/hwmon
      5 drivers/hwmon/occ
      4 drivers/hwmon/peci
     19 drivers/hwmon/pmbus
      8 drivers/hwtracing/intel_th
      1 drivers/hwtracing/stm
      9 drivers/i2c
    110 drivers/i2c/busses
      4 drivers/i2c/muxes
      1 drivers/i2c/muxes/pinctrl
     15 drivers/i3c/master
      5 drivers/i3c/master/mipi-i3c-hci
      2 drivers/iio
     82 drivers/iio/accel
    248 drivers/iio/adc
      6 drivers/iio/addac
      7 drivers/iio/amplifiers
      2 drivers/iio/cdc
     25 drivers/iio/chemical
      1 drivers/iio/common/cros_ec_sensors
      2 drivers/iio/common/ms_sensors
      5 drivers/iio/common/ssp_sensors
     80 drivers/iio/dac
      2 drivers/iio/dummy
     19 drivers/iio/frequency
     19 drivers/iio/gyro
     11 drivers/iio/humidity
      2 drivers/iio/humidity/common/ms_sensors
     23 drivers/iio/imu
      3 drivers/iio/imu/bmi160
      4 drivers/iio/imu/bmi270
      7 drivers/iio/imu/bmi323
      4 drivers/iio/imu/bno055
      7 drivers/iio/imu/inv_icm42600
      7 drivers/iio/imu/inv_mpu6050
     19 drivers/iio/imu/st_lsm6dsx
     79 drivers/iio/light
     24 drivers/iio/magnetometer
      3 drivers/iio/orientation
      3 drivers/iio/position
     11 drivers/iio/potentiometer
      1 drivers/iio/potentiostat
     34 drivers/iio/pressure
      2 drivers/iio/pressure/common/ms_sensors
     26 drivers/iio/proximity
      9 drivers/iio/resolver
     19 drivers/iio/temperature
      2 drivers/iio/temperature/common/ms_sensors
      2 drivers/iio/trigger
     32 drivers/infiniband/core
     11 drivers/infiniband/sw/siw
      4 drivers/infiniband/ulp/ipoib
      5 drivers/infiniband/ulp/iser
      6 drivers/infiniband/ulp/isert
     11 drivers/infiniband/ulp/rtrs
     10 drivers/infiniband/ulp/srp
      5 drivers/infiniband/ulp/srpt
      8 drivers/input
      1 drivers/input/gameport
     29 drivers/input/joystick
      2 drivers/input/joystick/iforce
     43 drivers/input/keyboard
     57 drivers/input/misc
     42 drivers/input/mouse
     40 drivers/input/rmi4
      8 drivers/input/serio
      5 drivers/input/tablet
    119 drivers/input/touchscreen
      2 drivers/interconnect
      1 drivers/interconnect/imx
      5 drivers/iommu
     15 drivers/iommu/iommufd
      2 drivers/ipack/devices
      6 drivers/irqchip
      9 drivers/isdn/hardware/mISDN
      4 drivers/isdn/mISDN
     56 drivers/leds
      2 drivers/leds/blink
      9 drivers/leds/flash
      9 drivers/leds/rgb
      5 drivers/leds/trigger
     19 drivers/mailbox
     97 drivers/md
     20 drivers/md/bcache
      9 drivers/md/persistent-data
      9 drivers/media/cec/core
      3 drivers/media/cec/i2c
      3 drivers/media/cec/platform/cec-gpio
      3 drivers/media/cec/platform/tegra
      8 drivers/media/cec/usb/extron-da-hd-4k-plus
      4 drivers/media/cec/usb/pulse8
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     11 include/linux/iio
      1 include/linux/iio/accel
      5 include/linux/iio/adc
      1 include/linux/iio/afe
     14 include/linux/iio/common
      2 include/linux/iio/dac
      5 include/linux/iio/frequency
      4 include/linux/iio/imu
      3 include/linux/input
      8 include/linux/lockd
      1 include/linux/mdio
     44 include/linux/mfd
      1 include/linux/mfd/abx500
     10 include/linux/mfd/arizona
      1 include/linux/mfd/da9052
      2 include/linux/mfd/da9063
      1 include/linux/mfd/da9150
      2 include/linux/mfd/madera
      1 include/linux/mfd/mt6358
      5 include/linux/mfd/samsung
      5 include/linux/mfd/wm831x
      7 include/linux/mfd/wm8994
     38 include/linux/mmc
     40 include/linux/mtd
     13 include/linux/netfilter
      6 include/linux/netfilter/ipset
      1 include/linux/netfilter_arp
      4 include/linux/netfilter_bridge
      1 include/linux/netfilter_ipv4
      3 include/linux/phy
      4 include/linux/pinctrl
     87 include/linux/platform_data
      1 include/linux/platform_data/txx9
      1 include/linux/platform_data/x86
      8 include/linux/power
      3 include/linux/pse-pd
     18 include/linux/regulator
      1 include/linux/remoteproc
      1 include/linux/reset
      2 include/linux/rtc
      4 include/linux/sched
      9 include/linux/soc/mediatek
      4 include/linux/soc/qcom
      8 include/linux/soc/ti
     13 include/linux/soundwire
     26 include/linux/spi
     16 include/linux/ssb
     17 include/linux/sunrpc
     82 include/linux/usb
     69 include/media
      2 include/media/davinci
      9 include/media/drv-intf
     24 include/media/i2c
      7 include/media/tpg
      7 include/memory
    627 include/net
     10 include/net/9p
     90 include/net/bluetooth
      5 include/net/caif
      6 include/net/libeth
     64 include/net/netfilter
     15 include/net/netfilter/../net/bridge
     15 include/net/netns
     20 include/net/nfc
      1 include/net/page_pool
      1 include/net/phonet
     40 include/net/sctp
     36 include/net/tc_act
     29 include/pcmcia
    101 include/rdma
     89 include/scsi
      2 include/soc/at91
      7 include/soc/fsl/qe
      2 include/soc/microchip
     17 include/soc/mscc
      5 include/soc/tegra
    162 include/sound
     31 include/target
     32 include/target/iscsi
      1 include/tools/lib/bpf
      6 include/trace/events/../fs/dlm
     15 include/trace/events/../net/bridge
     17 include/trace/events/../sound/soc/sof
      1 include/uapi/asm-generic
      2 include/uapi/drm
    216 include/uapi/linux
      1 include/uapi/linux/caif
      2 include/uapi/linux/can
      4 include/uapi/linux/dvb
      3 include/uapi/linux/hdlc
     58 include/uapi/linux/netfilter
      2 include/uapi/linux/netfilter/ipset
      3 include/uapi/linux/netfilter_arp
      6 include/uapi/linux/netfilter_bridge
      2 include/uapi/linux/netfilter_ipv4
      5 include/uapi/linux/netfilter_ipv6
      3 include/uapi/linux/tc_act
      2 include/uapi/linux/usb
      3 include/uapi/misc
      1 include/uapi/mtd
      2 include/uapi/rdma
      1 include/uapi/rdma/hfi
      1 include/uapi/scsi
     11 include/uapi/sound
      1 include/uapi/video
     28 include/ufs
     32 include/video
      2 init
     22 io_uring
      1 io_uring/fs
      1 io_uring/kernel/futex
      2 ipc
     23 kernel
     35 kernel/bpf
      4 kernel/bpf/cgroup
      1 kernel/bpf/tools/lib/bpf
      6 kernel/cgroup
      1 kernel/dma
      3 kernel/futex
      2 kernel/irq
      5 kernel/locking
      6 kernel/mm
      2 kernel/module
      1 kernel/printk
      4 kernel/rcu
     13 kernel/sched
      9 kernel/time
     30 lib
      5 lib/842
      1 lib/crc/tests
      1 lib/crypto
      1 lib/crypto/tests
      4 lib/kunit
      6 lib/mm
     14 lib/tests
      5 lib/xz
      3 lib/zlib_deflate
      4 lib/zstd/compress
      1 lib/zstd/decompress
     32 mm
      9 mm/damon
      2 net/6lowpan
      2 net/8021q
      6 net/9p
      7 net/atm
     15 net/atm/bridge
      1 net/ax25
     41 net/batman-adv
     28 net/bluetooth
      4 net/bluetooth/bnep
      4 net/bluetooth/hidp
      1 net/bluetooth/rfcomm
      1 net/bpf
     31 net/bridge
     15 net/bridge/netfilter
      8 net/caif
      3 net/can
      3 net/can/j1939
      5 net/ceph
     16 net/core
      8 net/devlink
      8 net/dsa
     16 net/ethtool
      1 net/ethtool/core
      1 net/handshake
      4 net/hsr
      1 net/ieee802154
     23 net/ipv4
      6 net/ipv6
     15 net/ipv6/bridge
      3 net/ipv6/ila
      1 net/key
      5 net/l2tp
    116 net/mac80211
    102 net/mac80211/tests
      3 net/mac802154
      4 net/mpls
     15 net/mptcp
     33 net/ncsi
     44 net/netfilter
     15 net/netfilter/bridge
     11 net/netfilter/ipset
      2 net/netfilter/ipvs
      1 net/netlabel
      3 net/netlink
     11 net/nfc
      6 net/nfc/hci
      4 net/nfc/nci
     11 net/openvswitch
      1 net/packet
      1 net/qrtr
     19 net/rds
      4 net/rfkill
     38 net/rxrpc
     53 net/sched
      2 net/sctp
      1 net/shaper/core
     38 net/smc
      5 net/sunrpc
      1 net/sunrpc/auth_gss
     37 net/tipc
      4 net/tls
      2 net/unix
      4 net/vmw_vsock
     24 net/wireless
     15 net/wireless/tests
     97 net/wireless/tests/mac80211
     10 net/xfrm
      6 samples/vfio-mdev
      6 security/apparmor/include
      3 security/integrity/evm
      1 security/integrity/ima
      1 security/keys
      7 security/landlock
      4 security/selinux
     12 security/selinux/include
     10 security/selinux/ss
     36 security/tomoyo
      1 security/yama
      3 sound/core
      3 sound/core/oss
      8 sound/core/seq
      9 sound/core/seq/oss
      9 sound/drivers
      1 sound/drivers/vx
      9 sound/firewire
      8 sound/firewire/bebob
      7 sound/firewire/bebob/.
      9 sound/firewire/dice
      8 sound/firewire/digi00x
      9 sound/firewire/fireface
      9 sound/firewire/fireface/.
      8 sound/firewire/fireworks
      8 sound/firewire/fireworks/.
      9 sound/firewire/motu
      8 sound/firewire/motu/.
     16 sound/firewire/oxfw
      9 sound/firewire/oxfw/.
     11 sound/firewire/tascam
     11 sound/firewire/tascam/.
     31 sound/hda/codecs
     14 sound/hda/codecs/cirrus
     18 sound/hda/codecs/common
      7 sound/hda/codecs/hdmi
     15 sound/hda/codecs/realtek
      2 sound/hda/codecs/realtek/side-codecs
      2 sound/hda/codecs/side-codecs
     19 sound/hda/common
      2 sound/hda/core
      2 sound/pci/ac97
      2 sound/pcmcia/pdaudiocf
      1 sound/soc/adi
      2 sound/soc/amd
     18 sound/soc/amd/sof/amd
      2 sound/soc/apple
      5 sound/soc/atmel
      1 sound/soc/atmel/codecs
      2 sound/soc/bcm
    335 sound/soc/codecs
      3 sound/soc/codecs/aw88395
      2 sound/soc/dwc
     25 sound/soc/fsl
      6 sound/soc/fsl/codecs
      1 sound/soc/generic
      1 sound/soc/hisilicon
      4 sound/soc/img
      3 sound/soc/intel/boards/codecs
      3 sound/soc/intel/keembay
      1 sound/soc/jz4740
      5 sound/soc/mediatek/common
      5 sound/soc/mediatek/mt8186
      3 sound/soc/mediatek/mt8186/codecs
      3 sound/soc/mediatek/mt8186/common
      5 sound/soc/mediatek/mt8188
      5 sound/soc/mediatek/mt8188/codecs
      3 sound/soc/mediatek/mt8188/common
      4 sound/soc/mediatek/mt8195
      1 sound/soc/mediatek/mt8195/codecs
      3 sound/soc/mediatek/mt8195/common
      9 sound/soc/mediatek/mt8365
      3 sound/soc/mediatek/mt8365/common
      2 sound/soc/meson
     10 sound/soc/qcom
      1 sound/soc/qcom/codecs
     17 sound/soc/qcom/qdsp6
      2 sound/soc/renesas
      1 sound/soc/renesas/rcar
      4 sound/soc/rockchip
      2 sound/soc/rockchip/codecs
      3 sound/soc/samsung
     12 sound/soc/samsung/codecs
     19 sound/soc/sof
     14 sound/soc/sof/imx
     17 sound/soc/sof/intel
     17 sound/soc/sof/mediatek
     17 sound/soc/sof/mediatek/mt8186
     17 sound/soc/sof/mediatek/mt8195
     11 sound/soc/sof/xtensa
      2 sound/soc/sprd
      4 sound/soc/starfive
      4 sound/soc/stm
      5 sound/soc/sunxi
      2 sound/soc/tegra
     16 sound/soc/ti
      2 sound/soc/uniphier
     10 sound/soc/usb
      4 sound/soc/xilinx
      1 sound/spi
     43 sound/usb
     10 sound/usb/6fire
      7 sound/usb/caiaq
      5 sound/usb/hiface
      3 sound/usb/line6
      5 sound/usb/misc
     44 sound/usb/qcom
      5 sound/usb/usx2y
      1 sound/virtio

Right, and that's the only transformation I mentioned.

That simplification should be amenable to further automation, if the first transformation is.

It's a matter of taste.

Right. That's why I said the list of struct members would need to be "manually pared down". But even that task may be too large to be feasible.

Right. That's why I advocated running the plug-in on a "normal build" not a contrived mass of #includes and structs.