cpumask: Fix invalid uniprocessor assumptions
cover cpumask: Fix invalid uniprocessor assumptions
On uniprocessor builds, it is currently assumed that any cpumask will contain the single CPU: cpu0. This assumption is used to provide optimised implementations.
The current assumption also appears to be wrong, by ignoring the fact that users can provide empty cpumask-s. This can result in bugs as explained in [1].
This series seeks to fix this assumption, allowing for masks that contain at most cpu0, i.e. are "1" or "0".
[1] https://lore.kernel.org/all/[email protected]/
Best,
Sander
---
To test these changes, I used the following code:
static void cpumask_test(const struct cpumask *mask)
{
int cpu;
if (cpumask_empty(mask))
pr_info("testing empty mask\n");
else
pr_info("testing non-empty mask\n");
pr_info("first cpu: %d\n", cpumask_first(mask));
pr_info("first zero: %d\n", cpumask_first_zero(mask));
pr_info("first and: %d\n", cpumask_first_and(mask, cpu_possible_mask));
pr_info("last cpu: %d\n", cpumask_last(mask));
pr_info("next cpu at -1: %d\n", cpumask_next(-1, mask));
pr_info("next cpu at 0: %d\n", cpumask_next(0, mask));
pr_info("next zero at -1: %d\n", cpumask_next_zero(-1, mask));
pr_info("next zero at 0: %d\n", cpumask_next_zero(0, mask));
pr_info("next and at -1: %d\n", cpumask_next_and(-1, mask, cpu_possible_mask));
pr_info("next and at 0: %d\n", cpumask_next_and(0, mask, cpu_possible_mask));
pr_info("next wrap at -1,false: %d\n", cpumask_next_wrap(-1, mask, 0, false));
pr_info("next wrap at 0,false: %d\n", cpumask_next_wrap(-1, mask, 0, false));
pr_info("next wrap at -1,true: %d\n", cpumask_next_wrap(-1, mask, 0, true));
pr_info("next wrap at 0,true: %d\n", cpumask_next_wrap(-1, mask, 0, true));
for_each_cpu(cpu, mask)
pr_info("for each: %d\n", cpu);
for_each_cpu_not(cpu, mask)
pr_info("for each not: %d\n", cpu);
for_each_cpu_wrap(cpu, mask, 0)
pr_info("for each wrap: %d\n", cpu);
for_each_cpu_and(cpu, mask, cpu_possible_mask)
pr_info("for each and: %d\n", cpu);
}static void run_tests()
{
cpumask_clear(&empty_mask);
cpumask_test(&empty_mask);
cpumask_test(cpu_possible_mask);
}On an unpatched build, with NR_CPUS=1:
[...] testing empty mask
[...] first cpu: 0
[...] first zero: 0
[...] first and: 0
[...] last cpu: 0
[...] next cpu at -1: 0
[...] next cpu at 0: 1
[...] next zero at -1: 0
[...] next zero at 0: 1
[...] next and at -1: 0
[...] next and at 0: 1
[...] next wrap at -1,false: 0
[...] next wrap at 0,false: 0
[...] next wrap at -1,true: 0
[...] next wrap at 0,true: 0
[...] for each: 0
[...] for each not: 0
[...] for each wrap: 0
[...] for each and: 0
[...] testing non-empty mask
[...] first cpu: 0
[...] first zero: 0
[...] first and: 0
[...] last cpu: 0
[...] next cpu at -1: 0
[...] next cpu at 0: 1
[...] next zero at -1: 0
[...] next zero at 0: 1
[...] next and at -1: 0
[...] next and at 0: 1
[...] next wrap at -1,false: 0
[...] next wrap at 0,false: 0
[...] next wrap at -1,true: 0
[...] next wrap at 0,true: 0
[...] for each: 0
[...] for each not: 0
[...] for each wrap: 0
[...] for each and: 0On a patched build, with NR_CPUS=1:
[...] testing empty mask
[...] first cpu: 1
[...] first zero: 0
[...] first and: 1
[...] last cpu: 1
[...] next cpu at -1: 1
[...] next cpu at 0: 1
[...] next zero at -1: 0
[...] next zero at 0: 1
[...] next and at -1: 1
[...] next and at 0: 1
[...] next wrap at -1,false: 1
[...] next wrap at 0,false: 1
[...] next wrap at -1,true: 1
[...] next wrap at 0,true: 1
[...] for each not: 0
[...] testing non-empty mask
[...] first cpu: 0
[...] first zero: 1
[...] first and: 0
[...] last cpu: 0
[...] next cpu at -1: 0
[...] next cpu at 0: 1
[...] next zero at -1: 1
[...] next zero at 0: 1
[...] next and at -1: 0
[...] next and at 0: 1
[...] next wrap at -1,false: 0
[...] next wrap at 0,false: 0
[...] next wrap at -1,true: 0
[...] next wrap at 0,true: 0
[...] for each: 0
[...] for each wrap: 0For reference, the generic implementation with NR_CPUS=2, CONFIG_SMP=y
[...] testing empty mask
[...] first cpu: 2
[...] first zero: 0
[...] first and: 2
[...] last cpu: 2
[...] next cpu at -1: 2
[...] next cpu at 0: 2
[...] next zero at -1: 0
[...] next zero at 0: 1
[...] next and at -1: 2
[...] next and at 0: 2
[...] next wrap at -1,false: 2
[...] next wrap at 0,false: 2
[...] next wrap at -1,true: 2
[...] next wrap at 0,true: 2
[...] for each not: 0
[...] testing non-empty mask
[...] first cpu: 0
[...] first zero: 1
[...] first and: 0
[...] last cpu: 0
[...] next cpu at -1: 0
[...] next cpu at 0: 2
[...] next zero at -1: 1
[...] next zero at 0: 1
[...] next and at -1: 0
[...] next and at 0: 2
[...] next wrap at -1,false: 0
[...] next wrap at 0,false: 0
[...] next wrap at -1,true: 2
[...] next wrap at 0,true: 2
[...] for each: 0
[...] for each wrap: 0
[...] for each and: 0Sander Vanheule (2):
cpumask: Fix invalid uniprocessor mask assumption
cpumask: Add UP optimised for_each_*_cpu loops
include/linux/cpumask.h | 42 +++++++++++++++++++++-------------------- 1 file changed, 22 insertions(+), 20 deletions(-)
Signature · 1 line
2.36.1
PATCH 1/2 cpumask: Fix invalid uniprocessor mask assumption
On uniprocessor builds, any CPU mask is assumed to contain exactly one CPU (cpu0). This ignores the existence of empty masks, resulting in incorrect behaviour for most of the implemented optimisations.
Replace the uniprocessor optimisations with implementations that also take into account empty masks. Also drop the incorrectly optimised for_each_cpu implementations and use the generic implementations in all cases.
--- include/linux/cpumask.h | 35 +++++++++++++++-------------------- 1 file changed, 15 insertions(+), 20 deletions(-)
include/linux/cpumask.h +15 -20
@@ -117,51 +117,54 @@ static __always_inline unsigned int cpumask_check(unsigned int cpu) }#if NR_CPUS ==1 -/* Uniprocessor. Assume all masks are "1". */ +/* Uniprocessor. Assume all valid masks are "1" or empty. */ static inline unsigned int cpumask_first (const struct cpumask *srcp) { -return 0 ; +return !(*cpumask_bits (srcp) &1 ); }static inline unsigned int cpumask_first_zero (const struct cpumask *srcp) { -return 0 ; +return *cpumask_bits (srcp) &1 ; }static inline unsigned int cpumask_first_and (const struct cpumask *srcp1,const struct cpumask *srcp2) { -return 0 ; +return !(*cpumask_bits (srcp1) & *cpumask_bits (srcp2) &1 ); }static inline unsigned int cpumask_last (const struct cpumask *srcp) { -return 0 ; +return cpumask_first (srcp); }/* Valid inputs for n are -1 and 0. */ static inline unsigned int cpumask_next (int n,const struct cpumask *srcp) { -return n+1 ; +return !!(n +1 +cpumask_first (srcp)); }static inline unsigned int cpumask_next_zero (int n,const struct cpumask *srcp) { -return n+1 ; +return !!(n +1 +cpumask_first_zero (srcp)); }static inline unsigned int cpumask_next_and (int n,const struct cpumask *srcp,const struct cpumask *andp) { -return n+1 ; +return !!(n +1 +cpumask_first_and (srcp, andp)); }static inline unsigned int cpumask_next_wrap (int n,const struct cpumask *mask,int start,bool wrap) { -/* cpu0 unless stop condition, wrap and at cpu0, then nr_cpumask_bits */ -return (wrap && n ==0 ); +/* +* nr_cpumask_bits at stop condition, wrap and at cpu0, or empty mask +* otherwise cpu0 +*/ +return (wrap && n ==0 ) ||cpumask_first (mask); }/* cpu must be a valid cpu, ie 0, so there's no other choice. */
@@ -186,14 +189,6 @@ static inline int cpumask_any_distribute(const struct cpumask *srcp) return cpumask_first(srcp); } -#define for_each_cpu (cpu, mask) \ - for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask) -#define for_each_cpu_not (cpu, mask) \ - for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask) -#define for_each_cpu_wrap (cpu, mask, start) \ - for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask, (void)(start)) -#define for_each_cpu_and (cpu, mask1, mask2) \ - for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask1, (void)mask2)#else /** * cpumask_first - get the first cpu in a cpumask
@@ -259,11 +254,13 @@ static inline unsigned int cpumask_next_zero(int n, const struct cpumask *srcp) }int __pure cpumask_next_and (int n,const struct cpumask *,const struct cpumask *); +extern int cpumask_next_wrap (int n,const struct cpumask *mask,int start,bool wrap);int __pure cpumask_any_but (const struct cpumask *mask,unsigned int cpu);unsigned int cpumask_local_spread (unsigned int i,int node);int cpumask_any_and_distribute (const struct cpumask *src1p,const struct cpumask *src2p);int cpumask_any_distribute (const struct cpumask *srcp); +#endif /* SMP */ /** * for_each_cpu - iterate over every cpu in a mask
@@ -289,7 +286,6 @@ int cpumask_any_distribute(const struct cpumask *srcp); (cpu) =cpumask_next_zero ((cpu), (mask)), \ (cpu) < nr_cpu_ids;) -extern int cpumask_next_wrap (int n,const struct cpumask *mask,int start,bool wrap);/** * for_each_cpu_wrap - iterate over every cpu in a mask, starting at a specified location
@@ -324,7 +320,6 @@ extern int cpumask_next_wrap(int n, const struct cpumask *mask, int start, boolfor ((cpu) =-1 ; \ (cpu) =cpumask_next_and ((cpu), (mask1), (mask2)), \ (cpu) < nr_cpu_ids;) -#endif /* SMP */ #define CPU_BITS_NONE \ { \
Signature · 1 line
2.36.1
Yury Norov
maintainerRe: [PATCH v1 1/2] cpumask: Fix invalid uniprocessor mask assumption
Quoted 19 lines · Sander Vanheule
On uniprocessor builds, any CPU mask is assumed to contain exactly one CPU (cpu0). This ignores the existence of empty masks, resulting in incorrect behaviour for most of the implemented optimisations.
Replace the uniprocessor optimisations with implementations that also take into account empty masks. Also drop the incorrectly optimised
for_each_cpuimplementations and use the generic implementations in all cases.Signed-off-by Sander Vanheule <[email protected]>--- include/linux/cpumask.h | 35 +++++++++++++++-------------------- 1 file changed, 15 insertions(+), 20 deletions(-)include/linux/cpumask.h +0
-0
include/linux/cpumask.h static __always_inline unsigned int cpumask_check(unsigned int cpu) · line 124
Quoted 10 lines
include/linux/cpumask.h +2
-2@@ -117,51 +117,54 @@ static __always_inline unsigned int cpumask_check(unsigned int cpu) }#if NR_CPUS ==1 -/* Uniprocessor. Assume all masks are "1". */ +/* Uniprocessor. Assume all valid masks are "1" or empty. */ static inline unsigned int cpumask_first (const struct cpumask *srcp) { -return 0 ; +return !(*cpumask_bits (srcp) &1 ); }
I think, you can just drop this '#if NR_CPUS == 1' part and rely on SMP versions. find_first_bit() is optimized for short bitmaps, so I expect no overhead comparing to this.
Quoted 69 lines
include/linux/cpumask.h +11
-16static inline unsigned int cpumask_first_zero (const struct cpumask *srcp) { -return 0 ; +return *cpumask_bits (srcp) &1 ; }static inline unsigned int cpumask_first_and (const struct cpumask *srcp1,const struct cpumask *srcp2) { -return 0 ; +return !(*cpumask_bits (srcp1) & *cpumask_bits (srcp2) &1 ); }static inline unsigned int cpumask_last (const struct cpumask *srcp) { -return 0 ; +return cpumask_first (srcp); }/* Valid inputs for n are -1 and 0. */ static inline unsigned int cpumask_next (int n,const struct cpumask *srcp) { -return n+1 ; +return !!(n +1 +cpumask_first (srcp)); }static inline unsigned int cpumask_next_zero (int n,const struct cpumask *srcp) { -return n+1 ; +return !!(n +1 +cpumask_first_zero (srcp)); }static inline unsigned int cpumask_next_and (int n,const struct cpumask *srcp,const struct cpumask *andp) { -return n+1 ; +return !!(n +1 +cpumask_first_and (srcp, andp)); }static inline unsigned int cpumask_next_wrap (int n,const struct cpumask *mask,int start,bool wrap) { -/* cpu0 unless stop condition, wrap and at cpu0, then nr_cpumask_bits */ -return (wrap && n ==0 ); +/* +* nr_cpumask_bits at stop condition, wrap and at cpu0, or empty mask +* otherwise cpu0 +*/ +return (wrap && n ==0 ) ||cpumask_first (mask); }/* cpu must be a valid cpu, ie 0, so there's no other choice. */ @@ -186,14 +189,6 @@ static inline int cpumask_any_distribute(const struct cpumask *srcp) return cpumask_first(srcp); } -#define for_each_cpu (cpu, mask) \ - for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask) -#define for_each_cpu_not (cpu, mask) \ - for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask) -#define for_each_cpu_wrap (cpu, mask, start) \ - for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask, (void)(start)) -#define for_each_cpu_and (cpu, mask1, mask2) \ - for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask1, (void)mask2)#else /** * cpumask_first - get the first cpu in a cpumask
include/linux/cpumask.h static inline unsigned int cpumask_next_zero(int n, const struct cpumask *srcp) · line 257
Quoted 4 lines
include/linux/cpumask.h +1
-0@@ -259,11 +254,13 @@ static inline unsigned int cpumask_next_zero(int n, const struct cpumask *srcp) }int __pure cpumask_next_and (int n,const struct cpumask *,const struct cpumask *); +extern int cpumask_next_wrap (int n,const struct cpumask *mask,int start,bool wrap);
is this extern needed?
Thanks,
Yury
Quoted 27 lines
include/linux/cpumask.h +1
-2int __pure cpumask_any_but (const struct cpumask *mask,unsigned int cpu);unsigned int cpumask_local_spread (unsigned int i,int node);int cpumask_any_and_distribute (const struct cpumask *src1p,const struct cpumask *src2p);int cpumask_any_distribute (const struct cpumask *srcp); +#endif /* SMP */ /** * for_each_cpu - iterate over every cpu in a mask @@ -289,7 +286,6 @@ int cpumask_any_distribute(const struct cpumask *srcp); (cpu) =cpumask_next_zero ((cpu), (mask)), \ (cpu) < nr_cpu_ids;) -extern int cpumask_next_wrap (int n,const struct cpumask *mask,int start,bool wrap);/** * for_each_cpu_wrap - iterate over every cpu in a mask, starting at a specified location @@ -324,7 +320,6 @@ extern int cpumask_next_wrap(int n, const struct cpumask *mask, int start, boolfor ((cpu) =-1 ; \ (cpu) =cpumask_next_and ((cpu), (mask1), (mask2)), \ (cpu) < nr_cpu_ids;) -#endif /* SMP */ #define CPU_BITS_NONE \ { \Signature · 1 line
2.36.1
Re: [PATCH v1 1/2] cpumask: Fix invalid uniprocessor mask assumption
Quoted 27 lines · Yury Norov
Quoted 26 lines · Sander Vanheule
On uniprocessor builds, any CPU mask is assumed to contain exactly one CPU (cpu0). This ignores the existence of empty masks, resulting in incorrect behaviour for most of the implemented optimisations.
Replace the uniprocessor optimisations with implementations that also take into account empty masks. Also drop the incorrectly optimised
for_each_cpuimplementations and use the generic implementations in all cases.Signed-off-by Sander Vanheule <[email protected]>---
include/linux/cpumask.h | 35 +++++++++++++++--------------------1 file changed, 15 insertions(+), 20 deletions(-)include/linux/cpumask.h +0
-0@@ -117,51 +117,54 @@ static __always_inline unsigned int cpumask_check(unsigned int cpu) }
#ifNR_CPUS== 1include/linux/cpumask.h +1
-1-/* Uniprocessor. Assume all masks are "1". */ +/* Uniprocessor. Assume all valid masks are "1" or empty. */ static inline unsigned int
cpumask_first(const struct cpumask *srcp)
Quoted 8 lines
Quoted 4 lines
{
- return 0;
+ return !(*cpumask_bits(srcp) & 1);
}
I think, you can just drop this '#ifNR_CPUS== 1' part and rely on SMP versions.find_first_bit()is optimized for short bitmaps, so I expect no overhead comparing to this.
I was initially planning to just drop the UP versions, but that means I have to include lib/cpumask.o on all builds. I'll have another look at this.
[...]
Quoted 7 lines
Quoted 7 lines
static inline unsigned int cpumask_next_wrap(int n, const struct cpumask *mask,
int start, bool wrap) {
- /* cpu0 unless stop condition, wrap and at cpu0, thennr_cpumask_bits*/
Quoted 8 lines
Quoted 8 lines
- return (wrap && n == 0);
+ /*
+ *nr_cpumask_bitsat stop condition, wrap and at cpu0, or empty mask
+ * otherwise cpu0
+ */
+ return (wrap && n == 0) ||cpumask_first(mask);
}
My tests contained a copy-paste error, so I missed a case. The results currently still differ from the SMP implementation though, so this looks like another reason to just drop the UP optimised version.
Quoted 20 lines
Quoted 20 lines
/* cpu must be a valid cpu, ie 0, so there's no other choice. */include/linux/cpumask.h +0
-0@@ -186,14 +189,6 @@ static inline int cpumask_any_distribute(const struct cpumask *srcp) return cpumask_first(srcp); }include/linux/cpumask.h +0
-8-#define for_each_cpu (cpu, mask) \ - for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask) -#define for_each_cpu_not (cpu, mask) \ - for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask) -#define for_each_cpu_wrap (cpu, mask, start) \ - for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask, (void)(start)) -#define for_each_cpu_and (cpu, mask1, mask2) \ - for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask1, (void)mask2)#else
/**
*cpumask_first- get the first cpu in a cpumaskinclude/linux/cpumask.h +0
-0@@ -259,11 +254,13 @@ static inline unsigned int cpumask_next_zero(int n, const struct cpumask *srcp)
Quoted 8 lines
Quoted 6 lines
include/linux/cpumask.h +0
-0}
int__purecpumask_next_and(int n, const struct cpumask *, const struct cpumask *);
+extern intcpumask_next_wrap(int n, const struct cpumask *mask, int start, bool wrap);is this extern needed?
I don't think it is, but this is a line that I just moved around in the header. Like for the other functions, I could even add the __pure qualifier.
Thanks for the feedback,
Sander
PATCH 2/2 cpumask: Add UP optimised for_each_*_cpu loops
In reply to Sander VanheuleOn uniprocessor builds, the following loops will always run over a mask that contains one enabled CPU (cpu0):
- for_each_possible_cpu
- for_each_online_cpu
- for_each_present_cpu
Provide uniprocessor-specific macros for these loops, that always run exactly once.
--- include/linux/cpumask.h | 7 +++++++ 1 file changed, 7 insertions(+)
include/linux/cpumask.h +7 -0
@@ -806,9 +806,16 @@ extern const DECLARE_BITMAP(cpu_all_bits, NR_CPUS);/* First bits of cpu_bit_bitmap are in fact unset. */ #define cpu_none_mask to_cpumask(cpu_bit_bitmap[0]) +#if NR_CPUS ==1 +/* Uniprocessor: the possible/online/present masks are always "1" */ +#define for_each_possible_cpu (cpu) for ((cpu) = 0; (cpu) < 1; (cpu)++) +#define for_each_online_cpu (cpu) for ((cpu) = 0; (cpu) < 1; (cpu)++) +#define for_each_present_cpu (cpu) for ((cpu) = 0; (cpu) < 1; (cpu)++) +#else #define for_each_possible_cpu (cpu) for_each_cpu((cpu), cpu_possible_mask)#define for_each_online_cpu (cpu) for_each_cpu((cpu), cpu_online_mask)#define for_each_present_cpu (cpu) for_each_cpu((cpu), cpu_present_mask) +#endif /* Wrappers for arch boot code to manipulate normally-constant masks */ void init_cpu_present (const struct cpumask *src);
Signature · 1 line
2.36.1