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mm/slub: fix lockups on PREEMPT && !SMP kernels

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Commit 9aabf810a6 ("mm/slub: optimize alloc/free fastpath by removing
preemption on/off") introduced an occasional hang for kernels built with
CONFIG_PREEMPT && !CONFIG_SMP.

The problem is the following loop the patch introduced to
slab_alloc_node and slab_free:

    do {
        tid = this_cpu_read(s->cpu_slab->tid);
        c = raw_cpu_ptr(s->cpu_slab);
    } while (IS_ENABLED(CONFIG_PREEMPT) && unlikely(tid != c->tid));

GCC 4.9 has been observed to hoist the load of c and c->tid above the
loop for !SMP kernels (as in this case raw_cpu_ptr(x) is compile-time
constant and does not force a reload).  On arm64 the generated assembly
looks like:

         ldr     x4, [x0,#8]
  loop:
         ldr     x1, [x0,#8]
         cmp     x1, x4
         b.ne    loop

If the thread is preempted between the load of c->tid (into x1) and tid
(into x4), and an allocation or free occurs in another thread (bumping
the cpu_slab's tid), the thread will be stuck in the loop until
s->cpu_slab->tid wraps, which may be forever in the absence of
allocations/frees on the same CPU.

This patch changes the loop condition to access c->tid with READ_ONCE.
This ensures that the value is reloaded even when the compiler would
otherwise assume it could cache the value, and also ensures that the
load will not be torn.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Jesper Dangaard Brouer <brouer@redhat.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Steve Capper <steve.capper@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Mark Rutland 2015-03-25 15:55:23 -07:00 committed by Linus Torvalds
parent b0dc3a342a
commit 859b7a0e89
1 changed files with 4 additions and 2 deletions
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6
mm/slub.c
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@ -2449,7 +2449,8 @@ redo:
do {
tid = this_cpu_read(s->cpu_slab->tid);
c = raw_cpu_ptr(s->cpu_slab);
} while (IS_ENABLED(CONFIG_PREEMPT) && unlikely(tid != c->tid));
} while (IS_ENABLED(CONFIG_PREEMPT) &&
unlikely(tid != READ_ONCE(c->tid)));
/*
* Irqless object alloc/free algorithm used here depends on sequence
@ -2718,7 +2719,8 @@ redo:
do {
tid = this_cpu_read(s->cpu_slab->tid);
c = raw_cpu_ptr(s->cpu_slab);
} while (IS_ENABLED(CONFIG_PREEMPT) && unlikely(tid != c->tid));
} while (IS_ENABLED(CONFIG_PREEMPT) &&
unlikely(tid != READ_ONCE(c->tid)));
/* Same with comment on barrier() in slab_alloc_node() */
barrier();