Commit-ID: b7e4aadef28f217de8907eec60a964328797a2be Gitweb: https://git.kernel.org/tip/b7e4aadef28f217de8907eec60a964328797a2be Author: Andrea Parri <andrea.pa...@amarulasolutions.com> AuthorDate: Mon, 14 May 2018 16:01:27 -0700 Committer: Ingo Molnar <mi...@kernel.org> CommitDate: Tue, 15 May 2018 08:11:15 +0200
locking/spinlocks: Document the semantics of spin_is_locked() 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.pa...@amarulasolutions.com> Co-Developed-by: Alan Stern <st...@rowland.harvard.edu> Co-Developed-by: David Howells <dhowe...@redhat.com> Signed-off-by: Andrea Parri <andrea.pa...@amarulasolutions.com> Signed-off-by: Alan Stern <st...@rowland.harvard.edu> Signed-off-by: David Howells <dhowe...@redhat.com> Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com> Acked-by: Randy Dunlap <rdun...@infradead.org> Cc: Akira Yokosawa <aki...@gmail.com> Cc: Andrew Morton <a...@linux-foundation.org> Cc: Boqun Feng <boqun.f...@gmail.com> Cc: Jade Alglave <j.algl...@ucl.ac.uk> Cc: Linus Torvalds <torva...@linux-foundation.org> Cc: Luc Maranget <luc.maran...@inria.fr> Cc: Nicholas Piggin <npig...@gmail.com> Cc: Peter Zijlstra <pet...@infradead.org> Cc: Thomas Gleixner <t...@linutronix.de> Cc: Will Deacon <will.dea...@arm.com> Cc: linux-a...@vger.kernel.org Cc: parri.and...@gmail.com Link: http://lkml.kernel.org/r/1526338889-7003-1-git-send-email-paul...@linux.vnet.ibm.com Signed-off-by: Ingo Molnar <mi...@kernel.org> --- 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);
