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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: Will Deacon <will.deacon@arm.com>
Acked-by: Peter 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: Ingo Molnar <mingo@kernel.org>
71 lines
1.5 KiB
Text
71 lines
1.5 KiB
Text
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On atomic bitops.
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While our bitmap_{}() functions are non-atomic, we have a number of operations
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operating on single bits in a bitmap that are atomic.
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API
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---
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The single bit operations are:
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Non-RMW ops:
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test_bit()
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RMW atomic operations without return value:
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{set,clear,change}_bit()
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clear_bit_unlock()
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RMW atomic operations with return value:
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test_and_{set,clear,change}_bit()
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test_and_set_bit_lock()
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Barriers:
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smp_mb__{before,after}_atomic()
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All RMW atomic operations have a '__' prefixed variant which is non-atomic.
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SEMANTICS
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---------
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Non-atomic ops:
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In particular __clear_bit_unlock() suffers the same issue as atomic_set(),
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which is why the generic version maps to clear_bit_unlock(), see atomic_t.txt.
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RMW ops:
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The test_and_{}_bit() operations return the original value of the bit.
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ORDERING
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--------
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Like with atomic_t, the rule of thumb is:
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- non-RMW operations are unordered;
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- RMW operations that have no return value are unordered;
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- RMW operations that have a return value are fully ordered.
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- RMW operations that are conditional are unordered on FAILURE,
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otherwise the above rules apply. In the case of test_and_{}_bit() operations,
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if the bit in memory is unchanged by the operation then it is deemed to have
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failed.
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Except for a successful test_and_set_bit_lock() which has ACQUIRE semantics and
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clear_bit_unlock() which has RELEASE semantics.
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Since a platform only has a single means of achieving atomic operations
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the same barriers as for atomic_t are used, see atomic_t.txt.
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