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x86, bitops: note on __test_and_clear_bit atomicity 

__test_and_clear_bit is actually atomic with respect
to the local CPU. Add a note saying that KVM on x86
relies on this behaviour so people don't accidentaly break it.
Also warn not to rely on this in portable code.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
This commit is contained in:
Michael S. Tsirkin 2012-06-24 19:24:42 +03:00 committed by Avi Kivity
parent ab9cf4996b
commit d0a69d6321
1 changed files with 7 additions and 0 deletions
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7
arch/x86/include/asm/bitops.h
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@ -264,6 +264,13 @@ static inline int test_and_clear_bit(int nr, volatile unsigned long *addr)
* This operation is non-atomic and can be reordered.
* If two examples of this operation race, one can appear to succeed
* but actually fail. You must protect multiple accesses with a lock.
*
* Note: the operation is performed atomically with respect to
* the local CPU, but not other CPUs. Portable code should not
* rely on this behaviour.
* KVM relies on this behaviour on x86 for modifying memory that is also
* accessed from a hypervisor on the same CPU if running in a VM: don't change
* this without also updating arch/x86/kernel/kvm.c
*/
static inline int __test_and_clear_bit(int nr, volatile unsigned long *addr)
{