Messages in this thread Patch in this message | | | From | Mark Rutland <> | Subject | [PATCHv2] mm/slub: fix lockups on PREEMPT && !SMP kernels | Date | Tue, 17 Mar 2015 12:15:33 +0000 |
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Commit 9aabf810a67cd97e ("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:
ffffffc00016d3c4: f9400404 ldr x4, [x0,#8] ffffffc00016d3c8: f9400401 ldr x1, [x0,#8] ffffffc00016d3cc: eb04003f cmp x1, x4 ffffffc00016d3d0: 54ffffc1 b.ne ffffffc00016d3c8 <slab_alloc_node.constprop.82+0x30>
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: Andrew Morton <akpm@linux-foundation.org> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: 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: Linus Torvalds <torvalds@linux-foundation.org> Cc: Pekka Enberg <penberg@kernel.org> Cc: Steve Capper <steve.capper@linaro.org> --- mm/slub.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)
Since v1 [1]: * Do not erroneously remove the loop
[1] lkml.kernel.org/r/1426261632-8911-1-git-send-email-mark.rutland@arm.com
diff --git a/mm/slub.c b/mm/slub.c index 6832c4e..82c4737 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -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(); -- 1.9.1
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