linux-kernel - Re: [PATCH v4 1/3] locking: Document the semantics of spin_is

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Message-Id: <20180406210741.GF3948@linux.vnet.ibm.com>
Date:   Fri, 6 Apr 2018 14:07:41 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     Andrea Parri <andrea.parri@...rulasolutions.com>,
        linux-kernel@...r.kernel.org,
        Alan Stern <stern@...land.harvard.edu>,
        David Howells <dhowells@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Akira Yokosawa <akiyks@...il.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v4 1/3] locking: Document the semantics of
 spin_is_locked()

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.parri@...rulasolutions.com>
> > Co-Developed-by: Alan Stern <stern@...land.harvard.edu>
> > Co-Developed-by: David Howells <dhowells@...hat.com>
> > Signed-off-by: Andrea Parri <andrea.parri@...rulasolutions.com>
> > Signed-off-by: Alan Stern <stern@...land.harvard.edu>
> > Signed-off-by: David Howells <dhowells@...hat.com>
> > Cc: Will Deacon <will.deacon@....com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Boqun Feng <boqun.feng@...il.com>
> > Cc: Nicholas Piggin <npiggin@...il.com>
> > Cc: Jade Alglave <j.alglave@....ac.uk>
> > Cc: Luc Maranget <luc.maranget@...ia.fr>
> > Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> > Cc: Akira Yokosawa <akiyks@...il.com>
> > Cc: Ingo Molnar <mingo@...nel.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
>