diff options
| author | Michael S. Tsirkin <mst@redhat.com> | 2012-06-24 19:24:42 +0300 |
|---|---|---|
| committer | Avi Kivity <avi@redhat.com> | 2012-06-25 12:38:35 +0300 |
| commit | d0a69d6321ca759bb8d47803d06ba8571ab42d07 (patch) | |
| tree | 6b94990e0b4d43251740fa60deef29148bce9d58 | |
| parent | ab9cf4996bb989983e73da894b8dd0239aa2c3c2 (diff) | |
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>
| -rw-r--r-- | arch/x86/include/asm/bitops.h | 7 |
1 files changed, 7 insertions, 0 deletions
diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h index a6983b277220..72f5009deb5a 100644 --- a/arch/x86/include/asm/bitops.h +++ b/arch/x86/include/asm/bitops.h @@ -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) { |