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There are several places in the kernel where wait_on_bit is not followed by a memory barrier (for example, in drivers/md/dm-bufio.c:new_read). On architectures with weak memory ordering, it may happen that memory accesses that follow wait_on_bit are reordered before wait_on_bit and they may return invalid data. Fix this class of bugs by introducing a new function "test_bit_acquire" that works like test_bit, but has acquire memory ordering semantics. Signed-off-by: Mikulas Patocka <mpatocka@redhat.com> Acked-by: Will Deacon <will@kernel.org> Cc: stable@vger.kernel.org Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
70 lines
1.4 KiB
Plaintext
70 lines
1.4 KiB
Plaintext
=============
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Atomic bitops
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=============
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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 fully ordered.
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Except for a successful test_and_set_bit_lock() which has ACQUIRE semantics,
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clear_bit_unlock() which has RELEASE semantics and test_bit_acquire which has
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ACQUIRE 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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