Messages in this thread Patch in this message | | | Date | Wed, 16 May 2012 14:46:04 +0300 | From | "Michael S. Tsirkin" <> | Subject | [PATCHv4 2/5] 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> --- 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 c9c70ea..86f3a1e 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) { -- MST
| |