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Re: [RFC][PATCH RT 0/3] RT: Fix trylock deadlock without msleep() hack

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  Re: [RFC][PATCH RT 0/3] RT: Fix trylock deadlock without msleep()  hack Ingo Molnar <mingo@kernel.org> - 2015-09-08 09:40 +0200
    Re: [RFC][PATCH RT 0/3] RT: Fix trylock deadlock without msleep()  hack Thomas Gleixner <tglx@linutronix.de> - 2015-09-08 10:20 +0200
      Re: [RFC][PATCH RT 0/3] RT: Fix trylock deadlock without msleep()  hack Ingo Molnar <mingo@kernel.org> - 2015-09-14 12:00 +0200

#1220548 — Re: [RFC][PATCH RT 0/3] RT: Fix trylock deadlock without msleep() hack

* Thomas Gleixner <tglx@linutronix.de> wrote:

> 3) sched_yield() makes me shudder
> 
>    CPU0			CPU1	
> 
>    taskA
>      lock(x->lock)
> 
>    preemption
>    taskC
> 			taskB
> 			  lock(y->lock);
> 			  x = y->x;
> 			  if (!try_lock(x->lock)) {
> 			    unlock(y->lock);
> 			    boost(taskA);
> 			    sched_yield();  <- returns immediately

So I'm still struggling with properly parsing the usecase.

If y->x might become invalid the moment we drop y->lock, what makes the 'taskA' 
use (after we've dropped y->lock) safe? Shouldn't we at least also have a 
task_get(taskA)/task_put(taskA) reference count, to make sure the boosted task 
stays around?

And if we are into getting reference counts, why not solve it at a higher level 
and get a reference count to 'x' to make sure it's safe to use? Then we could do:

        lock(y->lock);
retry:
	x = y->x;
        if (!trylock(x->lock)) {
		get_ref(x->count)
                unlock(y->lock);
                lock(x->lock);
                lock(y->lock);
		put_ref(x->count);
		if (y->x != x) { /* Retry if 'x' got dropped meanwhile */
			unlock(x->lock);
			goto retry;
		}
        }

Or so.

Note how much safer this sequence is, and still just as fast in the common case 
(which I suppose is the main motivation within dcache.c?).

Thanks,

	Ingo
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#1220590

On Tue, 8 Sep 2015, Ingo Molnar wrote:
> 
> * Thomas Gleixner <tglx@linutronix.de> wrote:
> 
> > 3) sched_yield() makes me shudder
> > 
> >    CPU0			CPU1	
> > 
> >    taskA
> >      lock(x->lock)
> > 
> >    preemption
> >    taskC
> > 			taskB
> > 			  lock(y->lock);
> > 			  x = y->x;
> > 			  if (!try_lock(x->lock)) {
> > 			    unlock(y->lock);
> > 			    boost(taskA);
> > 			    sched_yield();  <- returns immediately
> 
> So I'm still struggling with properly parsing the usecase.
> 
> If y->x might become invalid the moment we drop y->lock, what makes
> the 'taskA' use (after we've dropped y->lock) safe? Shouldn't we at
> least also have a task_get(taskA)/task_put(taskA) reference count,
> to make sure the boosted task stays around?

Stevens trylock_and_boost() function makes sure that taskA cannot go
away while doing the boosting. It's a bug in my pseudo code, but that
does not make the issue above going away.
 
> And if we are into getting reference counts, why not solve it at a
> higher level and get a reference count to 'x' to make sure it's safe
> to use? Then we could do:
>
>         lock(y->lock);
> retry:
> 	x = y->x;
>         if (!trylock(x->lock)) {
> 		get_ref(x->count)
>                 unlock(y->lock);
>                 lock(x->lock);
>                 lock(y->lock);
> 		put_ref(x->count);
> 		if (y->x != x) { /* Retry if 'x' got dropped meanwhile */
> 			unlock(x->lock);
> 			goto retry;
> 		}
>         }
> 
> Or so.

In the case of dcache::dentry_kill() we probably do not have to take
refcounts and it might be actually counterproductive to do so. y->x,
i.e. dentry->parent, cannot vanish under us, if I understand the life
time rules correctly.

Aside of that, yes, I was thinking about a similar scheme for
that. I need some more time to grok all the rules there :)

Thanks,

	tglx


 
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#1223992

* Thomas Gleixner <tglx@linutronix.de> wrote:

> > And if we are into getting reference counts, why not solve it at a higher 
> > level and get a reference count to 'x' to make sure it's safe to use? Then we 
> > could do:
> >
> >         lock(y->lock);
> > retry:
> >         x = y->x;
> >         if (!trylock(x->lock)) {
> >                 get_ref(x->count)
> >                 unlock(y->lock);
> >                 lock(x->lock);
> >                 lock(y->lock);
> >                 put_ref(x->count);
> >                 if (y->x != x) { /* Retry if 'x' got dropped meanwhile */
> >                         unlock(x->lock);
> >                         goto retry;
> >                 }
> >          }
> > 
> > Or so.
> 
> In the case of dcache::dentry_kill() we probably do not have to take refcounts 
> and it might be actually counterproductive to do so. y->x, i.e. dentry->parent, 
> cannot vanish under us, if I understand the life time rules correctly.

Ok, that's even better.

> Aside of that, yes, I was thinking about a similar scheme for that. I need some 
> more time to grok all the rules there :)

Ok, great! :-)

I really don't think we need a new locking primitive - and with something like the 
above we could improve the code upstream as well and make it scale better in some 
scenarios, right?

Thanks,

	Ingo
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