Re: [PATCH v2 1/3] locking: Document the semantics of spin_is_locked()

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Apr 02, 2018 at 10:03:22AM -0400, Alan Stern wrote:
> On Sun, 1 Apr 2018, 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().
> > 
> > [1] https://marc.info/?l=linux-kernel&m=151981440005264&w=2
> > 
> > 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>
> > Cc: Ingo Molnar <mingo@redhat.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.
> 
> This is a good addition.  But please remove the parenthetical phrase.  
> Or if you prefer to keep it, at least remove the parentheses.

Unless someone objects or proposes a different course of action, I will
make this change in -rcu.

							Thanx, Paul

> Alan
> 
> > + */
> >  static __always_inline int spin_is_locked(spinlock_t *lock)
> >  {
> >  	return raw_spin_is_locked(&lock->rlock);
> > 
>