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

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Date:   Sun, 8 Apr 2018 14:14:46 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Andrea Parri <andrea.parri@...rulasolutions.com>
Cc:     Randy Dunlap <rdunlap@...radead.org>, 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 11:58:25PM +0200, Andrea Parri wrote:
> On Fri, Apr 06, 2018 at 02:08:16PM -0700, Randy Dunlap wrote:
> > On 04/06/2018 02:07 PM, Paul E. McKenney wrote:
> > > 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?
> > 
> > Please change this patch to current doc syntax.
> > Any changes to kernel-doc syntax would come later.

Are you sure?

$ git grep "\* Returns:" | wc -l
2470
$ git grep "\* Return:" | wc -l
4144

Looks like more than a third of them are already "Returns:".  ;-)

> Paul: I understand that you're going to do this change "in place"; please
> let me know if I'm wrong/if you need a new submission.

If Randy is certain that he would like to continue propagating
this grammatical infelicity, sure.  ;-)

							Thanx, Paul

> Thanks,
>   Andrea
> 
> 
> > 
> > Thanks.
> > 
> > > 							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
> > >>
> > > 
> > 
> > 
> > -- 
> > ~Randy
>