Re: [PATCH -v4 6/8] locking/mutex: Restructure wait loop

[Peter Zijlstra]

On Mon, Oct 17, 2016 at 12:44:49PM +0200, Peter Zijlstra wrote:
> On Thu, Oct 13, 2016 at 04:17:21PM +0100, Will Deacon wrote:
> > Hi Peter,
> > 
> > I'm struggling to get my head around the handoff code after this change...
> > 
> > On Fri, Oct 07, 2016 at 04:52:49PM +0200, Peter Zijlstra wrote:
> > > --- a/kernel/locking/mutex.c
> > > +++ b/kernel/locking/mutex.c
> > > @@ -631,13 +631,21 @@ __mutex_lock_common(struct mutex *lock,
> > >  
> > >  	lock_contended(&lock->dep_map, ip);
> > >  
> > > +	set_task_state(task, state);
> > >  	for (;;) {
> > > +		/*
> > > +		 * Once we hold wait_lock, we're serialized against
> > > +		 * mutex_unlock() handing the lock off to us, do a trylock
> > > +		 * before testing the error conditions to make sure we pick up
> > > +		 * the handoff.
> > > +		 */
> > >  		if (__mutex_trylock(lock, first))
> > > -			break;
> > > +			goto acquired;
> > >  
> > >  		/*
> > > -		 * got a signal? (This code gets eliminated in the
> > > -		 * TASK_UNINTERRUPTIBLE case.)
> > > +		 * Check for signals and wound conditions while holding
> > > +		 * wait_lock. This ensures the lock cancellation is ordered
> > > +		 * against mutex_unlock() and wake-ups do not go missing.
> > >  		 */
> > >  		if (unlikely(signal_pending_state(state, task))) {
> > >  			ret = -EINTR;
> > > @@ -650,16 +658,27 @@ __mutex_lock_common(struct mutex *lock,
> > >  				goto err;
> > >  		}
> > >  
> > > -		__set_task_state(task, state);
> > >  		spin_unlock_mutex(&lock->wait_lock, flags);
> > >  		schedule_preempt_disabled();
> > > -		spin_lock_mutex(&lock->wait_lock, flags);
> > >  
> > >  		if (!first && __mutex_waiter_is_first(lock, &waiter)) {
> > >  			first = true;
> > >  			__mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
> > >  		}
> > > +
> > > +		set_task_state(task, state);
> > 
> > With this change, we no longer hold the lock wit_hen we set the task
> > state, and it's ordered strictly *after* setting the HANDOFF flag.
> > Doesn't that mean that the unlock code can see the HANDOFF flag, issue
> > the wakeup, but then we come in and overwrite the task state?
> > 
> > I'm struggling to work out whether that's an issue, but it certainly
> > feels odd and is a change from the previous behaviour.
> 
> Right, so I think the code is fine, since in that case the
> __mutex_trylock() must see the handoff and we'll break the loop and
> (re)set the state to RUNNING.
> 
> But you're right in that its slightly odd. I'll reorder them and put the
> set_task_state() above the !first thing.


Humm,.. we might actually rely on this order, since the MB implied by
set_task_state() is the only thing that separates the store of
__mutex_set_flag() from the load of __mutex_trylock(), and those should
be ordered I think.

Argh, completely messed up my brain. I'll not touch it and think on this
again tomorrow.