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Commit 61e02392 authored by Will Deacon's avatar Will Deacon Committed by Ingo Molnar
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locking/atomic/bitops: Document and clarify ordering semantics for failed test_and_{}_bit() 



A test_and_{}_bit() operation fails if the value of the bit is such that
the modification does not take place. For example, if test_and_set_bit()
returns 1. In these cases, follow the behaviour of cmpxchg and allow the
operation to be unordered. This also applies to test_and_set_bit_lock()
if the lock is found to be be taken already.

Signed-off-by: default avatarWill Deacon <will.deacon@arm.com>
Acked-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1518528619-20049-1-git-send-email-will.deacon@arm.com


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 11dc1322

Documentation/atomic_bitops.txt

+6 −1
Original line number Diff line number Diff line
@@ -58,7 +58,12 @@ Like with atomic_t, the rule of thumb is: 


 - RMW operations that have a return value are fully ordered.



Except for test_and_set_bit_lock() which has ACQUIRE semantics and

 - RMW operations that are conditional are unordered on FAILURE,

   otherwise the above rules apply. In the case of test_and_{}_bit() operations,

   if the bit in memory is unchanged by the operation then it is deemed to have

   failed.



Except for a successful test_and_set_bit_lock() which has ACQUIRE semantics and

clear_bit_unlock() which has RELEASE semantics.



Since a platform only has a single means of achieving atomic operations

include/asm-generic/bitops/lock.h

+2 −1
Original line number Diff line number Diff line
@@ -7,7 +7,8 @@ 
 * @nr: Bit to set

 * @addr: Address to count from

 *

 * This operation is atomic and provides acquire barrier semantics.

 * This operation is atomic and provides acquire barrier semantics if

 * the returned value is 0.

 * It can be used to implement bit locks.

 */

#define test_and_set_bit_lock(nr, addr)	test_and_set_bit(nr, addr)