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 <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Jesper Dangaard Brouer <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: Steve Capper <[email protected]>
---
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/[email protected]
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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