linux-kernel - Re: [PATCH] locking/mutex: Set and clear owner using WRITE

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Date:	Fri, 20 May 2016 15:09:45 -0700
From:	Jason Low <jason.low2@....com>
To:	Waiman Long <waiman.long@....com>
Cc:	jason.low2@...com, Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Davidlohr Bueso <dave@...olabs.net>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Terry Rudd <terry.rudd@....com>,
	Scott J Norton <scott.norton@....com>
Subject: Re: [PATCH] locking/mutex: Set and clear owner using WRITE_ONCE()

On Fri, 2016-05-20 at 16:27 -0400, Waiman Long wrote:
> On 05/19/2016 06:23 PM, Jason Low wrote:
> > The mutex owner can get read and written to without the wait_lock.
> > Use WRITE_ONCE when setting and clearing the owner field in order
> > to avoid optimizations such as store tearing. This avoids
> > situations where the owner field gets written to with multiple
> > stores and another thread could concurrently read and use a
> > partially written owner value.
> >
> > Signed-off-by: Jason Low<jason.low2@....com>
> > ---
> >   kernel/locking/mutex.h | 10 ++++++++--
> >   1 file changed, 8 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/locking/mutex.h b/kernel/locking/mutex.h
> > index 5cda397..469b61e 100644
> > --- a/kernel/locking/mutex.h
> > +++ b/kernel/locking/mutex.h
> > @@ -17,14 +17,20 @@
> >   		__list_del((waiter)->list.prev, (waiter)->list.next)
> >
> >   #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
> > +/*
> > + * The mutex owner can get read and written to locklessly.
> > + * We should use WRITE_ONCE() when writing the owner value to
> > + * avoid store tearing, otherwise, a thread could potentially
> > + * read a partially written and incomplete owner value.
> > + */
> >   static inline void mutex_set_owner(struct mutex *lock)
> >   {
> > -	lock->owner = current;
> > +	WRITE_ONCE(lock->owner, current);
> >   }
> >
> >   static inline void mutex_clear_owner(struct mutex *lock)
> >   {
> > -	lock->owner = NULL;
> > +	WRITE_ONCE(lock->owner, NULL);
> >   }
> >   #else
> >   static inline void mutex_set_owner(struct mutex *lock)
> 
> I think mutex-debug.h also needs similar changes for completeness.

Good point, I will add the changes to those functions in the debug case
to this patch.

Thanks,
Jason