linux-kernel - [PATCH] mm: slub: Ensure that slab

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Date:	Tue, 8 Mar 2016 20:00:57 +0530
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	<linux-mm@...ck.org>
CC:	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Noam Camus <noamc@...hip.com>, <stable@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>,
	<linux-snps-arc@...ts.infradead.org>
Subject: [PATCH] mm: slub: Ensure that slab_unlock() is atomic

We observed livelocks on ARC SMP setup when running hackbench with SLUB.
This hardware configuration lacks atomic instructions (LLOCK/SCOND) thus
kernel resorts to a central @smp_bitops_lock to protect any R-M-W ops
suh as test_and_set_bit()

The spinlock itself is implemented using Atomic [EX]change instruction
which is always available.

The race happened when both cores tried to slab_lock() the same page.

   c1		    c0
-----------	-----------
slab_lock
		slab_lock
slab_unlock
		Not observing the unlock

This in turn happened because slab_unlock() doesn't serialize properly
(doesn't use atomic clear) with a concurrent running
slab_lock()->test_and_set_bit()

Cc: Christoph Lameter <cl@...ux.com>
Cc: Pekka Enberg <penberg@...nel.org>
Cc: David Rientjes <rientjes@...gle.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Noam Camus <noamc@...hip.com>
Cc: <stable@...r.kernel.org>
Cc: <linux-mm@...ck.org>
Cc: <linux-kernel@...r.kernel.org>
Cc: <linux-snps-arc@...ts.infradead.org>
Signed-off-by: Vineet Gupta <vgupta@...opsys.com>
---
 mm/slub.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/slub.c b/mm/slub.c
index d8fbd4a6ed59..b7d345a508dc 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -345,7 +345,7 @@ static __always_inline void slab_lock(struct page *page)
 static __always_inline void slab_unlock(struct page *page)
 {
 	VM_BUG_ON_PAGE(PageTail(page), page);
-	__bit_spin_unlock(PG_locked, &page->flags);
+	bit_spin_unlock(PG_locked, &page->flags);
 }
 
 static inline void set_page_slub_counters(struct page *page, unsigned long counters_new)
-- 
2.5.0