[PATCHv7 4/8] x86/bitops: note on __test_and_clear_bit atomicity

Michael S. Tsirkin
2012-06-14T13:52:54Z

__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 file changed, 7 insertions(+)

diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index 47f9eff..75e3787 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -266,6 +266,13 @@ 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