On Tue, Apr 03, 2018 at 04:23:07PM +0100, David Howells wrote: > Andrea Parri <andrea.pa...@amarulasolutions.com> wrote: > > > Sorry, but I don't understand your objection: are you suggesting to add > > something like "Always return 0 on !SMP" to the comment? what else? > > Something like that, possibly along with a warning that this might not be what > you want. You might actually want it to return true on !SMP, it depends on > what you're using it for.
I ended up with the following revision. I hesitated on whether to refer to 'include/linux/spinlock_up.h' or not, but in the end I decided to not include the reference. Please let me know what you think about this. Andrea >From 85f2d12d4ad9769cc9f69cc5f447fdb8c5ed4d14 Mon Sep 17 00:00:00 2001 From: Andrea Parri <andrea.pa...@amarulasolutions.com> Date: Tue, 3 Apr 2018 21:23:07 +0200 Subject: [PATCH v3 1/3] locking: 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(). [1] https://marc.info/?l=linux-kernel&m=151981440005264&w=2 Signed-off-by: Andrea Parri <andrea.pa...@amarulasolutions.com> Cc: Alan Stern <st...@rowland.harvard.edu> Cc: Will Deacon <will.dea...@arm.com> Cc: Peter Zijlstra <pet...@infradead.org> Cc: Boqun Feng <boqun.f...@gmail.com> Cc: Nicholas Piggin <npig...@gmail.com> Cc: David Howells <dhowe...@redhat.com> Cc: Jade Alglave <j.algl...@ucl.ac.uk> Cc: Luc Maranget <luc.maran...@inria.fr> Cc: "Paul E. McKenney" <paul...@linux.vnet.ibm.com> Cc: Akira Yokosawa <aki...@gmail.com> Cc: Ingo Molnar <mi...@redhat.com> --- include/linux/spinlock.h | 14 ++++++++++++++ 1 file changed, 14 insertions(+) diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h index 4894d322d2584..636a4436191c1 100644 --- a/include/linux/spinlock.h +++ b/include/linux/spinlock.h @@ -380,6 +380,20 @@ 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. + * + * Return: 1, if @lock is (found to be) locked; 0, otherwise. + * + * Remark that this primitve can return a fixed value + * under certain !SMP configurations. + */ static __always_inline int spin_is_locked(spinlock_t *lock) { return raw_spin_is_locked(&lock->rlock); -- 2.7.4 > > David
