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SubjectRe: [PATCH 4/4] futex: convert hash_bucket locks to raw_spinlock_t
On Mon, 12 Jul 2010, Darren Hart wrote:
> On 07/10/2010 12:41 PM, Mike Galbraith wrote:
> > On Fri, 2010-07-09 at 15:33 -0700, Darren Hart wrote:
> > > > Out of curiosity, what's wrong with holding his pi_lock across the
> > > > wakeup? He can _try_ to block, but can't until pi state is stable.
> > > >
> > > > I presume there's a big fat gotcha that's just not obvious to futex
> > > > locking newbie :)
>
> Nor to some of us that have been engrossed in futexes for the last couple
> years! I discussed the pi_lock across the wakeup issue with Thomas. While this
> fixes the problem for this particular failure case, it doesn't protect
> against:
>
> <tglx> assume the following:
> <tglx> t1 is on the condvar
> <tglx> t2 does the requeue dance and t1 is now blocked on the outer futex
> <tglx> t3 takes hb->lock for a futex in the same bucket
> <tglx> t2 wakes due to signal/timeout
> <tglx> t2 blocks on hb->lock
>
> You are likely to have not hit the above scenario because you only had one
> condvar, so the hash_buckets were not heavily shared and you weren't likely to
> hit:
>
> <tglx> t3 takes hb->lock for a futex in the same bucket
>
>
> I'm going to roll up a patchset with your (Mike) spin_trylock patch and run it
> through some tests. I'd still prefer a way to detect early wakeup without
> having to grab the hb->lock(), but I haven't found it yet.
>
> + while(!spin_trylock(&hb->lock))
> + cpu_relax();
> ret = handle_early_requeue_pi_wakeup(hb, &q, &key2, to);
> spin_unlock(&hb->lock);

And this is nasty as it will create unbound priority inversion :(

We discussed another solution on IRC in meantime:

in futex_wait_requeue_pi()

futex_wait_queue_me(hb, &q, to);

raw_spin_lock(current->pi_lock);
if (current->pi_blocked_on) {
/*
* We know that we can only be blocked on the outer futex
* so we can skip the early wakeup check
*/
raw_spin_unlock(current->pi_lock);
ret = 0;
} else {
current->pi_blocked_on = PI_WAKEUP_INPROGRESS;
raw_spin_unlock(current->pi_lock);
spin_lock(&hb->lock);
ret = handle_early_requeue_pi_wakeup();
....
spin_lock(&hb->lock);
}
Now in the rtmutex magic we need in task_blocks_on_rt_mutex():

raw_spin_lock(task->pi_lock);

/*
* Add big fat comment why this is only relevant to futex
* requeue_pi
*/
if (task != current && task->pi_blocked_on == PI_WAKEUP_INPROGRESS) {
raw_spin_lock(task->pi_lock);
/*
* Returning 0 here is fine. the requeue code is just going to
* move the futex_q to the other bucket, but that'll be fixed
* up in handle_early_requeue_pi_wakeup()
*/
return 0;
}
Thanks,

tglx




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