From d0a69d6321ca759bb8d47803d06ba8571ab42d07 Mon Sep 17 00:00:00 2001 From: "Michael S. Tsirkin" Date: Sun, 24 Jun 2012 19:24:42 +0300 Subject: [PATCH] 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 Signed-off-by: Avi Kivity --- arch/x86/include/asm/bitops.h | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h index a6983b27722..72f5009deb5 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) { -- 2.46.0