| Message ID | 1518528619-20049-1-git-send-email-will.deacon@arm.com |
|---|---|
| State | Accepted |
| Commit | 61e02392d3c7ecac1f91c0a90a8043d67e081846 |
| Headers | show |
| Series | atomic/bitops: Clarify ordering semantics for failed test_and_{}_bit() | expand |
On Tue, Feb 13, 2018 at 01:30:19PM +0000, Will Deacon wrote: > 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. You also looked at a bunch of users, right? And while you found some dodgy ones, they were not more broken because of this IIRC. Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> > Cc: Peter Zijlstra <peterz@infradead.org> > Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> > Signed-off-by: Will Deacon <will.deacon@arm.com> > --- > Documentation/atomic_bitops.txt | 7 ++++++- > include/asm-generic/bitops/lock.h | 3 ++- > 2 files changed, 8 insertions(+), 2 deletions(-) > > diff --git a/Documentation/atomic_bitops.txt b/Documentation/atomic_bitops.txt > index 5550bfdcce5f..be70b32c95d9 100644 > --- a/Documentation/atomic_bitops.txt > +++ b/Documentation/atomic_bitops.txt > @@ -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 > diff --git a/include/asm-generic/bitops/lock.h b/include/asm-generic/bitops/lock.h > index bc397573c43a..67ab280ad134 100644 > --- a/include/asm-generic/bitops/lock.h > +++ b/include/asm-generic/bitops/lock.h > @@ -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) > -- > 2.1.4 >
diff --git a/Documentation/atomic_bitops.txt b/Documentation/atomic_bitops.txt index 5550bfdcce5f..be70b32c95d9 100644 --- a/Documentation/atomic_bitops.txt +++ b/Documentation/atomic_bitops.txt @@ -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 diff --git a/include/asm-generic/bitops/lock.h b/include/asm-generic/bitops/lock.h index bc397573c43a..67ab280ad134 100644 --- a/include/asm-generic/bitops/lock.h +++ b/include/asm-generic/bitops/lock.h @@ -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)
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. Cc: Peter Zijlstra <peterz@infradead.org> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> Signed-off-by: Will Deacon <will.deacon@arm.com> --- Documentation/atomic_bitops.txt | 7 ++++++- include/asm-generic/bitops/lock.h | 3 ++- 2 files changed, 8 insertions(+), 2 deletions(-) -- 2.1.4