On Fri, Apr 06, 2018 at 09:47:39PM +0200, Andrea Parri wrote:
> 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>
> 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: 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...@kernel.org>
Queued in place of its predecessor, thank you all!
Thanx, Paul
> ---
> include/linux/spinlock.h | 18 ++++++++++++++++++
> 1 file changed, 18 insertions(+)
>
> diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
> index 4894d322d2584..1e8a464358384 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);
> --
> 2.7.4
>
