From: tip-bot for Peter Zijlstra <tipbot@zytor.com>
To: linux-tip-commits@vger.kernel.org
Cc: rientjes@google.com, jejb@parisc-linux.org, mingo@kernel.org,
deller@gmx.de, cl@linux.com, akpm@linux-foundation.org,
linux-kernel@vger.kernel.org, Vineet.Gupta1@synopsys.com,
peterz@infradead.org, iamjoonsoo.kim@lge.com, hpa@zytor.com,
tglx@linutronix.de, penberg@kernel.org,
torvalds@linux-foundation.org, paulmck@linux.vnet.ibm.com,
noamc@ezchip.com
Subject: [tip:locking/urgent] bitops: Do not default to __clear_bit() for __clear_bit_unlock()
Date: Mon, 21 Mar 2016 04:16:33 -0700 [thread overview]
Message-ID: <tip-f75d48644c56a31731d17fa693c8175328957e1d@git.kernel.org> (raw)
In-Reply-To: <20160309114054.GJ6356@twins.programming.kicks-ass.net>
Commit-ID: f75d48644c56a31731d17fa693c8175328957e1d
Gitweb: http://git.kernel.org/tip/f75d48644c56a31731d17fa693c8175328957e1d
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Wed, 9 Mar 2016 12:40:54 +0100
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 21 Mar 2016 10:50:48 +0100
bitops: Do not default to __clear_bit() for __clear_bit_unlock()
__clear_bit_unlock() is a special little snowflake. While it carries the
non-atomic '__' prefix, it is specifically documented to pair with
test_and_set_bit() and therefore should be 'somewhat' atomic.
Therefore the generic implementation of __clear_bit_unlock() cannot use
the fully non-atomic __clear_bit() as a default.
If an arch is able to do better; is must provide an implementation of
__clear_bit_unlock() itself.
Specifically, this came up as a result of hackbench livelock'ing in
slab_lock() on ARC with SMP + SLUB + !LLSC.
The issue was incorrect pairing of atomic ops.
slab_lock() -> bit_spin_lock() -> test_and_set_bit()
slab_unlock() -> __bit_spin_unlock() -> __clear_bit()
The non serializing __clear_bit() was getting "lost"
80543b8e: ld_s r2,[r13,0] <--- (A) Finds PG_locked is set
80543b90: or r3,r2,1 <--- (B) other core unlocks right here
80543b94: st_s r3,[r13,0] <--- (C) sets PG_locked (overwrites unlock)
Fixes ARC STAR 9000817404 (and probably more).
Reported-by: Vineet Gupta <Vineet.Gupta1@synopsys.com>
Tested-by: Vineet Gupta <Vineet.Gupta1@synopsys.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Helge Deller <deller@gmx.de>
Cc: James E.J. Bottomley <jejb@parisc-linux.org>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Noam Camus <noamc@ezchip.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/20160309114054.GJ6356@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
include/asm-generic/bitops/lock.h | 14 +++++++-------
1 file changed, 7 insertions(+), 7 deletions(-)
diff --git a/include/asm-generic/bitops/lock.h b/include/asm-generic/bitops/lock.h
index c30266e..8ef0ccb 100644
--- a/include/asm-generic/bitops/lock.h
+++ b/include/asm-generic/bitops/lock.h
@@ -29,16 +29,16 @@ do { \
* @nr: the bit to set
* @addr: the address to start counting from
*
- * This operation is like clear_bit_unlock, however it is not atomic.
- * It does provide release barrier semantics so it can be used to unlock
- * a bit lock, however it would only be used if no other CPU can modify
- * any bits in the memory until the lock is released (a good example is
- * if the bit lock itself protects access to the other bits in the word).
+ * A weaker form of clear_bit_unlock() as used by __bit_lock_unlock(). If all
+ * the bits in the word are protected by this lock some archs can use weaker
+ * ops to safely unlock.
+ *
+ * See for example x86's implementation.
*/
#define __clear_bit_unlock(nr, addr) \
do { \
- smp_mb(); \
- __clear_bit(nr, addr); \
+ smp_mb__before_atomic(); \
+ clear_bit(nr, addr); \
} while (0)
#endif /* _ASM_GENERIC_BITOPS_LOCK_H_ */
next prev parent reply other threads:[~2016-03-21 11:17 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-08 14:30 [PATCH] mm: slub: Ensure that slab_unlock() is atomic Vineet Gupta
2016-03-08 15:00 ` Christoph Lameter
2016-03-08 15:46 ` Vineet Gupta
2016-03-08 20:40 ` Christoph Lameter
2016-03-09 6:43 ` Vineet Gupta
2016-03-09 10:13 ` Peter Zijlstra
2016-03-09 10:31 ` Peter Zijlstra
2016-03-09 11:12 ` Vineet Gupta
2016-03-09 11:00 ` Vineet Gupta
2016-03-09 11:40 ` Peter Zijlstra
2016-03-09 11:53 ` Vineet Gupta
2016-03-09 12:22 ` Peter Zijlstra
2016-03-14 8:05 ` Vineet Gupta
2016-03-21 11:16 ` tip-bot for Peter Zijlstra [this message]
2016-03-09 13:22 ` Vineet Gupta
2016-03-09 14:51 ` Peter Zijlstra
2016-03-10 5:51 ` Vineet Gupta
2016-03-10 9:10 ` Peter Zijlstra
2016-03-08 15:32 ` Vlastimil Babka
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