__TEST_AND_CLEAR_BIT(9) Basic C Library Functions __TEST_AND_CLEAR_BIT(9)NAME
__test_and_clear_bit - Clear a bit and return its old value
SYNOPSIS
int __test_and_clear_bit(int nr, volatile unsigned long * addr);
ARGUMENTS
nr
Bit to clear
addr
Address to count from
DESCRIPTION
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
COPYRIGHT Kernel Hackers Manual 3.10 June 2014 __TEST_AND_CLEAR_BIT(9)
Check Out this Related Man Page
ATOMIC_CAS(3) BSD Library Functions Manual ATOMIC_CAS(3)NAME
atomic_cas, atomic_cas_32, atomic_cas_uint, atomic_cas_ulong, atomic_cas_ptr, atomic_cas_64, atomic_cas_32_ni, atomic_cas_uint_ni,
atomic_cas_ulong_ni, atomic_cas_ptr_ni, atomic_cas_64_ni -- atomic compare-and-swap operations
SYNOPSIS
#include <sys/atomic.h>
uint32_t
atomic_cas_32(volatile uint32_t *ptr, uint32_t old, uint32_t new);
unsigned int
atomic_cas_uint(volatile unsigned int *ptr, unsigned int old, unsigned int new);
unsigned long
atomic_cas_ulong(volatile unsigned long *ptr, unsigned long old, unsigned long new);
void *
atomic_cas_ptr(volatile void *ptr, void *old, void *new);
uint64_t
atomic_cas_64(volatile uint64_t *ptr, uint64_t old, uint64_t new);
uint32_t
atomic_cas_32_ni(volatile uint32_t *ptr, uint32_t old, uint32_t new);
unsigned int
atomic_cas_uint_ni(volatile unsigned int *ptr, unsigned int old, unsigned int new);
unsigned long
atomic_cas_ulong_ni(volatile unsigned long *ptr, unsigned long old, unsigned long new);
void *
atomic_cas_ptr_ni(volatile void *ptr, void *old, void *new);
uint64_t
atomic_cas_64_ni(volatile uint64_t *ptr, uint64_t old, uint64_t new);
DESCRIPTION
The atomic_cas family of functions perform a compare-and-swap operation in an atomic fashion. The value of the variable referenced by ptr is
compared against old. If the values are equal, new is stored in the variable referenced by ptr.
The old value of the variable referenced by ptr is always returned regardless of whether or not the new value was stored. Applications can
test for success of the operation by comparing the return value to the value passed as old; if they are equal then the new value was stored.
The non-interlocked variants, *_ni(), guarantee atomicity within the same CPU with respect to interrupts and preemption. For example, they
are suitable for synchronizing compare-and-swap operations on a variable shared by a thread and an interrupt that are bound to the same CPU.
The *_ni() variants are not atomic with respect to different CPUs. *_ni() variants should avoid the interprocessor synchronization overhead
of the standard compare-and-swap operations.
The 64-bit variants of these functions are available only on platforms that can support atomic 64-bit memory access. Applications can check
for the availability of 64-bit atomic memory operations by testing if the pre-processor macro __HAVE_ATOMIC64_OPS is defined.
SEE ALSO atomic_ops(3)HISTORY
The atomic_cas functions first appeared in NetBSD 5.0.
NOTES
On some architectures, a *_ni() variant is merely an alias for the corresponding standard compare-and-swap operation. While the non-inter-
locked variant behaves correctly on those architectures, it does not avoid the interprocessor synchronization overhead.
BSD February 11, 2010 BSD