forked from Minki/linux
61e02392d3
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>
72 lines
1.5 KiB
Plaintext
72 lines
1.5 KiB
Plaintext
|
|
On atomic bitops.
|
|
|
|
|
|
While our bitmap_{}() functions are non-atomic, we have a number of operations
|
|
operating on single bits in a bitmap that are atomic.
|
|
|
|
|
|
API
|
|
---
|
|
|
|
The single bit operations are:
|
|
|
|
Non-RMW ops:
|
|
|
|
test_bit()
|
|
|
|
RMW atomic operations without return value:
|
|
|
|
{set,clear,change}_bit()
|
|
clear_bit_unlock()
|
|
|
|
RMW atomic operations with return value:
|
|
|
|
test_and_{set,clear,change}_bit()
|
|
test_and_set_bit_lock()
|
|
|
|
Barriers:
|
|
|
|
smp_mb__{before,after}_atomic()
|
|
|
|
|
|
All RMW atomic operations have a '__' prefixed variant which is non-atomic.
|
|
|
|
|
|
SEMANTICS
|
|
---------
|
|
|
|
Non-atomic ops:
|
|
|
|
In particular __clear_bit_unlock() suffers the same issue as atomic_set(),
|
|
which is why the generic version maps to clear_bit_unlock(), see atomic_t.txt.
|
|
|
|
|
|
RMW ops:
|
|
|
|
The test_and_{}_bit() operations return the original value of the bit.
|
|
|
|
|
|
ORDERING
|
|
--------
|
|
|
|
Like with atomic_t, the rule of thumb is:
|
|
|
|
- non-RMW operations are unordered;
|
|
|
|
- RMW operations that have no return value are unordered;
|
|
|
|
- RMW operations that have a return value are fully ordered.
|
|
|
|
- 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
|
|
the same barriers as for atomic_t are used, see atomic_t.txt.
|
|
|