linux-kernel - Re: [PATCH -v5 10/14] futex: Pull rt_mutex_futex

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Message-ID: <20170307180106.GF3312@twins.programming.kicks-ass.net>
Date:   Tue, 7 Mar 2017 19:01:06 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     mingo@...nel.org, juri.lelli@....com, rostedt@...dmis.org,
        xlpang@...hat.com, bigeasy@...utronix.de,
        linux-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com,
        jdesfossez@...icios.com, bristot@...hat.com, dvhart@...radead.org
Subject: Re: [PATCH -v5 10/14] futex: Pull rt_mutex_futex_unlock() out from
 under hb->lock

On Tue, Mar 07, 2017 at 03:08:17PM +0100, Thomas Gleixner wrote:
> On Sat, 4 Mar 2017, Peter Zijlstra wrote:
> > @@ -1035,6 +1037,9 @@ static int attach_to_pi_state(u32 __user
> >  	 * has dropped the hb->lock in between queue_me() and unqueue_me_pi(),
> >  	 * which in turn means that futex_lock_pi() still has a reference on
> >  	 * our pi_state.
> > +	 *
> > +	 * IOW, we cannot race against the unlocked put_pi_state() in
> > +	 * futex_unlock_pi().
> 
> That 'IOW' made my head spin for a while. I rather prefer to spell it out
> more explicitely:
> 
> 	 * The waiter holding a reference on @pi_state protects also
>          * against the unlocked put_pi_state() in futex_unlock_pi(),
>          * futex_lock_pi() and futex_wait_requeue_pi() as it cannot go to 0
>          * and consequentely free pi state before we can take a reference
>          * ourself.

Right you are. After staring at this for too damn long one tends to
forget what 'obvious' means.

> 
> >  	 */
> >  	WARN_ON(!atomic_read(&pi_state->refcount));
> >  
> > @@ -1378,47 +1383,33 @@ static void mark_wake_futex(struct wake_
> >  	smp_store_release(&q->lock_ptr, NULL);
> >  }
> >  
> > -static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *top_waiter,
> > -			 struct futex_hash_bucket *hb)
> 
> Please add a comment, that the caller must hold a reference on @pi_state

Will do.

> > +static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_pi_state *pi_state)
> >  {
> >  	u32 uninitialized_var(curval), newval;
> > +	struct task_struct *new_owner;
> > +	bool deboost = false;
> >  	DEFINE_WAKE_Q(wake_q);
> >  	int ret = 0;
> >  
> >  	raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
> >  	new_owner = rt_mutex_next_owner(&pi_state->pi_mutex);
> >  	if (!new_owner) {
> > +		/*
> > +		 * Since we held neither hb->lock nor wait_lock when coming
> > +		 * into this function, we could have raced with futex_lock_pi()
> > +		 * such that it will have removed the waiter that brought us
> > +		 * here.
> 
> Hmm. That's not entirely correct. There are two cases:
> 
>      lock_pi()
> 	queue_me() <- Makes it visible as waiter in the hash bucket
> 	unlock(hb->lock)
> 
>   [1]
> 
> 	rtmutex_futex_lock()
> 
>   [2]
>   
> 	lock(hb->lock)
> 
> Both [1] and [2] are valid reasons why the top waiter is not a rtmutex
> waiter.

Correct, I've even drawn similar state pictures elsewhere in this
series. I'll update.