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    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


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