Man page 9 : __test_and_clear_bit

__test_and_clear_bit - Clear a bit and return its old value

int __test_and_clear_bit(int  nr , volatile unsigned long *  addr );

nr
    Bit to clear

addr
    Address to count from

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.

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

donAqt change this without also updating arch/x86/kernel/kvm.c