Patch "powerpc/mm: Ensure IRQs are off in switch_mm()" has been added to the 4.11-stable tree

This is a note to let you know that I've just added the patch titled

powerpc/mm: Ensure IRQs are off in switch_mm()

to the 4.11-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
powerpc-mm-ensure-irqs-are-off-in-switch_mm.patch
and it can be found in the queue-4.11 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: David Gibson <[email protected]>
Date: Wed, 19 Apr 2017 16:38:26 +1000
Subject: powerpc/mm: Ensure IRQs are off in switch_mm()

From: David Gibson <[email protected]>

commit 9765ad134a00a01cbcc69c78ff6defbfad209bc5 upstream.

powerpc expects IRQs to already be (soft) disabled when switch_mm() is called, as made clear in the commit message of 9c1e105238c4 ("powerpc: Allow perf_counters to access user memory at interrupt time").

Aside from any race conditions that might exist between switch_mm() and an IRQ, there is also an unconditional hard_irq_disable() in switch_slb(). If that isn't followed at some point by an IRQ enable then interrupts will remain disabled until we return to userspace.

It is true that when switch_mm() is called from the scheduler IRQs are off, but not when it's called by use_mm(). Looking closer we see that last year in commit f98db6013c55 ("sched/core: Add switch_mm_irqs_off() and use it in the scheduler") this was made more explicit by the addition of switch_mm_irqs_off() which is now called by the scheduler, vs switch_mm() which is used by use_mm().

Arguably it is a bug in use_mm() to call switch_mm() in a different context than it expects, but fixing that will take time.

This was discovered recently when vhost started throwing warnings such as:

  BUG: sleeping function called from invalid context at kernel/mutex.c:578
  in_atomic(): 0, irqs_disabled(): 1, pid: 10768, name: vhost-10760
  no locks held by vhost-10760/10768.
  irq event stamp: 10
  hardirqs last  enabled at (9):  _raw_spin_unlock_irq+0x40/0x80
  hardirqs last disabled at (10): switch_slb+0x2e4/0x490
  softirqs last  enabled at (0):  copy_process+0x5e8/0x1260
  softirqs last disabled at (0):  (null)
  Call Trace:
    show_stack+0x88/0x390 (unreliable)
    dump_stack+0x30/0x44
    __might_sleep+0x1c4/0x2d0
    mutex_lock_nested+0x74/0x5c0
    cgroup_attach_task_all+0x5c/0x180
    vhost_attach_cgroups_work+0x58/0x80 [vhost]
    vhost_worker+0x24c/0x3d0 [vhost]
    kthread+0xec/0x100
    ret_from_kernel_thread+0x5c/0xd4

Prior to commit 04b96e5528ca ("vhost: lockless enqueuing") (Aug 2016) the vhost_worker() would do a spin_unlock_irq() not long after calling use_mm(), which had the effect of reenabling IRQs. Since that commit removed the locking in vhost_worker() the body of the vhost_worker() loop now runs with interrupts off causing the warnings.

This patch addresses the problem by making the powerpc code mirror the x86 code, ie. we disable interrupts in switch_mm(), and optimise the scheduler case by defining switch_mm_irqs_off().

---
 arch/powerpc/include/asm/mmu_context.h |   17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

@@ -70,8 +70,9 @@ extern void drop_cop(unsigned long acop,
  * switch_mm is the entry point called from the architecture independent
  * code in kernel/sched/core.c
  */
-static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
-			     struct task_struct *tsk)
+static inline void switch_mm_irqs_off(struct mm_struct *prev,
+				      struct mm_struct *next,
+				      struct task_struct *tsk)
 {
 	/* Mark this context has been used on the new CPU */
 	if (!cpumask_test_cpu(smp_processor_id(), mm_cpumask(next)))

@@ -110,6 +111,18 @@ static inline void switch_mm(struct mm_s
 	switch_mmu_context(prev, next, tsk);
 }
 
+static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
+			     struct task_struct *tsk)
+{
+	unsigned long flags;
+
+	local_irq_save(flags);
+	switch_mm_irqs_off(prev, next, tsk);
+	local_irq_restore(flags);
+}
+#define switch_mm_irqs_off switch_mm_irqs_off
+
+
 #define deactivate_mm(tsk,mm)	do { } while (0)
 
 /*

Patches currently in stable-queue which might be from [email protected] are

queue-4.11/powerpc-mm-ensure-irqs-are-off-in-switch_mm.patch