From: Andrea Parri <andrea.parri@xxxxxxxxxxxxxxxxxxxx> There appeared to be a certain, recurrent uncertainty concerning the semantics of spin_is_locked(), likely a consequence of the fact that this semantics remains undocumented or that it has been historically linked to the (likewise unclear) semantics of spin_unlock_wait(). A recent auditing [1] of the callers of the primitive confirmed that none of them are relying on particular ordering guarantees; document this semantics by adding a docbook header to spin_is_locked(). Also, describe behaviors specific to certain CONFIG_SMP=n builds. [1] https://marc.info/?l=linux-kernel&m=151981440005264&w=2 https://marc.info/?l=linux-kernel&m=152042843808540&w=2 https://marc.info/?l=linux-kernel&m=152043346110262&w=2 Co-Developed-by: Andrea Parri <andrea.parri@xxxxxxxxxxxxxxxxxxxx> Co-Developed-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> Co-Developed-by: David Howells <dhowells@xxxxxxxxxx> Signed-off-by: Andrea Parri <andrea.parri@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> Signed-off-by: David Howells <dhowells@xxxxxxxxxx> Cc: Will Deacon <will.deacon@xxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Boqun Feng <boqun.feng@xxxxxxxxx> Cc: Nicholas Piggin <npiggin@xxxxxxxxx> Cc: Jade Alglave <j.alglave@xxxxxxxxx> Cc: Luc Maranget <luc.maranget@xxxxxxxx> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> Cc: Akira Yokosawa <akiyks@xxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> Acked-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx> --- include/linux/spinlock.h | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h index 4894d322d258..1e8a46435838 100644 --- a/include/linux/spinlock.h +++ b/include/linux/spinlock.h @@ -380,6 +380,24 @@ static __always_inline int spin_trylock_irq(spinlock_t *lock) raw_spin_trylock_irqsave(spinlock_check(lock), flags); \ }) +/** + * spin_is_locked() - Check whether a spinlock is locked. + * @lock: Pointer to the spinlock. + * + * This function is NOT required to provide any memory ordering + * guarantees; it could be used for debugging purposes or, when + * additional synchronization is needed, accompanied with other + * constructs (memory barriers) enforcing the synchronization. + * + * Returns: 1 if @lock is locked, 0 otherwise. + * + * Note that the function only tells you that the spinlock is + * seen to be locked, not that it is locked on your CPU. + * + * Further, on CONFIG_SMP=n builds with CONFIG_DEBUG_SPINLOCK=n, + * the return value is always 0 (see include/linux/spinlock_up.h). + * Therefore you should not rely heavily on the return value. + */ static __always_inline int spin_is_locked(spinlock_t *lock) { return raw_spin_is_locked(&lock->rlock); -- 2.5.2
