[PATCHv2] mm/slub: fix lockups on PREEMPT && !SMP kernels

[Mark Rutland]

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@xxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Pekka Enberg <penberg@xxxxxxxxxx>
Cc: Steve Capper <steve.capper@xxxxxxxxxx>
---
 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@xxxxxxx

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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