Re: safety of *mutex_unlock() (Was: [BUG] signal: sighand unprotected when accessed by /proc)

[Thomas Gleixner]

On Tue, 10 Jun 2014, Oleg Nesterov wrote:

> On 06/10, Thomas Gleixner wrote:
> >
> > On Mon, 9 Jun 2014, Steven Rostedt wrote:
> > > I think rtmutex has an
> > > issue with it too. Specifically in the slow_unlock case:
> > >
> > > 	if (!rt_mutex_has_waiters(lock)) {
> > > 		lock->owner = NULL;
> > > 		raw_spin_unlock(&lock->wait_lock);
> > > 		return;
> > > 	}
> >
> > Indeed. If the fast path is enabled we have that issue. Fortunately
> > there is a halfways reasonable solution for this.
> 
> Ah, yes, I missed that,
> 
> > +	while (!rt_mutex_has_waiters(lock)) {
> > +		/* Drops lock->wait_lock ! */
> > +		if (unlock_rt_mutex_safe(lock) == true)
> > +			return;
> > +		/* Relock the rtmutex and try again */
> > +		raw_spin_lock(&lock->wait_lock);
> >  	}
> 
> OK...
> 
> wakeup_next_waiter() does rt_mutex_set_owner(NULL) before we drop ->wait_lock,
> but this looks fine: we know that rt_mutex_has_waiters() can not become false
> until waiter->task takes this lock and does rt_mutex_dequeue(), so ->owner
> can't be NULL, right?

Correct.

> Perhaps it could simply do ->owner = RT_MUTEX_HAS_WAITERS to make this more
> clear...

Good point. The new owner can cleanup the mess.
 
> Off-topic question. I simply can't understand why rt_mutex_slowtrylock() checks
> rt_mutex_owner(lock) != current. This looks pointless, try_to_take_rt_mutex()
> always fails (correctly) if rt_mutex_owner() != NULL ? IOW, can't we simply
> remove this check or turn it into "if (!rt_mutex_owner(lock))" ?

Indeed.

Thanks,

	tglx
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