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Commit d0a69d63 authored by Michael S. Tsirkin's avatar Michael S. Tsirkin Committed by Avi Kivity
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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: default avatarMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: default avatarAvi Kivity <avi@redhat.com>
parent ab9cf499

arch/x86/include/asm/bitops.h

+7 −0
Original line number Diff line number Diff line
@@ -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)

{