On Fri, Apr 06, 2018 at 02:01:41PM -0700, Randy Dunlap wrote:
> On 04/06/2018 12:47 PM, 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>
> > ---
> > 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.
>
> Sorry, minor nit:
> s/Returns:/Return:/
> (according to kernel-doc.rst)
>
> although I agree that "Returns:" is better.
> [I should have changed that years ago.]
Agreed, English grammar and templates often seem to conflict.
So should we change this comment, or are you instead proposing to add
"Returns:" as valid kernel-doc?
Thanx, Paul
> > + *
> > + * 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);
> >
>
>
> --
> ~Randy
>
