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authorMichael S. Tsirkin <mst@redhat.com>2012-06-24 12:24:42 -0400
committerAvi Kivity <avi@redhat.com>2012-06-25 05:38:35 -0400
commitd0a69d6321ca759bb8d47803d06ba8571ab42d07 (patch)
tree6b94990e0b4d43251740fa60deef29148bce9d58 /arch/x86/include/asm/bitops.h
parentab9cf4996bb989983e73da894b8dd0239aa2c3c2 (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>
Diffstat (limited to 'arch/x86/include/asm/bitops.h')
-rw-r--r--arch/x86/include/asm/bitops.h7
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)
264 * This operation is non-atomic and can be reordered. 264 * This operation is non-atomic and can be reordered.
265 * If two examples of this operation race, one can appear to succeed 265 * If two examples of this operation race, one can appear to succeed
266 * but actually fail. You must protect multiple accesses with a lock. 266 * but actually fail. You must protect multiple accesses with a lock.
267 *
268 * Note: the operation is performed atomically with respect to
269 * the local CPU, but not other CPUs. Portable code should not
270 * rely on this behaviour.
271 * KVM relies on this behaviour on x86 for modifying memory that is also
272 * accessed from a hypervisor on the same CPU if running in a VM: don't change
273 * this without also updating arch/x86/kernel/kvm.c
267 */ 274 */
268static inline int __test_and_clear_bit(int nr, volatile unsigned long *addr) 275static inline int __test_and_clear_bit(int nr, volatile unsigned long *addr)
269{ 276{