Re: [PATCH] Documentation/locking: Document the semantics of spin_is_locked()

[Andrea Parri <parri.andrea@gmail.com>]

On Wed, Feb 28, 2018 at 10:56:32AM +0000, Will Deacon wrote:
> On Wed, Feb 28, 2018 at 11:39:32AM +0100, 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().
> > 
> > Document this semantics.
> > 
> > Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
> > Cc: Alan Stern <stern@rowland.harvard.edu>
> > Cc: Will Deacon <will.deacon@arm.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Boqun Feng <boqun.feng@gmail.com>
> > Cc: Nicholas Piggin <npiggin@gmail.com>
> > Cc: David Howells <dhowells@redhat.com>
> > Cc: Jade Alglave <j.alglave@ucl.ac.uk>
> > Cc: Luc Maranget <luc.maranget@inria.fr>
> > Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > Cc: Akira Yokosawa <akiyks@gmail.com>
> > ---
> >  include/linux/spinlock.h | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> > 
> > diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h
> > index 4894d322d2584..2639fdc9a916c 100644
> > --- a/include/linux/spinlock.h
> > +++ b/include/linux/spinlock.h
> > @@ -380,6 +380,17 @@ 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.
> > + */
> 
> I also don't think this is quite right, since the spin_is_locked check
> must be ordered after all prior lock acquisitions (to any lock) on the same
> CPU. That's why we have an smp_mb() in there on arm64 (see 38b850a73034f).

So, arm64 (and powerpc) complies to the semantics I _have_ in mind ...


> 
> So this is a change in semantics and we need to audit the users before
> proceeding. We should also keep spin_is_locked consistent with the versions
> for mutex, rwsem, bit_spin.

Well, strictly speaking, it isn't (given that the current semantics is,
as reported above, currently undocumented); for the same reason, cases
relying on anything more than _nothing_ (if any) are already broken ...

  Andrea


> 
> Will