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| author | Peter Zijlstra <peterz@infradead.org> | 2016-03-09 12:40:54 +0100 |
|---|---|---|
| committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2016-04-12 07:33:42 -0700 |
| commit | 3c2aed0537b998d2b003fbd7b2f83faa6b77b80b (patch) | |
| tree | 9992a232a3acf81d28f8ca5601bb95056a9c38c8 /include | |
| parent | 085bf9a35c8e102315881a4bd8d2596f8a767739 (diff) | |
| download | lwn-3c2aed0537b998d2b003fbd7b2f83faa6b77b80b.tar.gz lwn-3c2aed0537b998d2b003fbd7b2f83faa6b77b80b.zip | |
bitops: Do not default to __clear_bit() for __clear_bit_unlock()
commit f75d48644c56a31731d17fa693c8175328957e1d upstream.
__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>
Link: http://lkml.kernel.org/r/20160309114054.GJ6356@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'include')
| -rw-r--r-- | include/asm-generic/bitops/lock.h | 14 |
1 files changed, 7 insertions, 7 deletions
diff --git a/include/asm-generic/bitops/lock.h b/include/asm-generic/bitops/lock.h index c30266e94806..8ef0ccbf8167 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_ */ |