Re: [PATCH 4/4] futex: convert hash_bucket locks to raw_spinlock_t

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On 07/12/2010 08:09 PM, Mike Galbraith wrote:
On Mon, 2010-07-12 at 22:40 +0200, Thomas Gleixner wrote:
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 :(

Oh ma gawd, _it's a train_ :>

Seriously.

I have a fix. Cleaning it up as we speak, still hope to send out tonight.

--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
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