Messages in this thread Patch in this message | | | Date | Fri, 16 Feb 2018 11:21:00 +0100 | From | Peter Zijlstra <> | Subject | Re: [RFC PATCH 3/5] asm-generic/bitops/atomic.h: Rewrite using atomic_fetch_* |
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On Thu, Feb 15, 2018 at 06:20:49PM +0000, Will Deacon wrote: > On Thu, Feb 15, 2018 at 06:08:47PM +0100, Peter Zijlstra wrote: > > On Thu, Feb 15, 2018 at 03:29:33PM +0000, Will Deacon wrote: > > > +static inline void __clear_bit_unlock(unsigned int nr, > > > + volatile unsigned long *p) > > > +{ > > > + unsigned long old; > > > > > > + p += BIT_WORD(nr); > > > + old = READ_ONCE(*p); > > > + old &= ~BIT_MASK(nr); > > > + smp_store_release(p, old); > > > > This should be atomic_set_release() I think, for the special case where > > atomics are implemented with spinlocks, see the 'fun' case in > > Documentation/atomic_t.txt. > > My understanding of __clear_bit_unlock is that there is guaranteed to be > no concurrent accesses to the same word, so why would it matter whether > locks are used to implement atomics?
commit f75d48644c56a31731d17fa693c8175328957e1d Author: Peter Zijlstra <peterz@infradead.org> Date: Wed Mar 9 12:40:54 2016 +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>
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_ */
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