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Date: Tue, 10 Jun 2014 22:13:20 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Steven Rostedt <rostedt@goodmis.org>
cc: Oleg Nesterov <oleg@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>, Clark Williams <williams@redhat.com>
Subject: Re: safety of *mutex_unlock() (Was: [BUG] signal: sighand unprotected
 when accessed by /proc)
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On Tue, 10 Jun 2014, Thomas Gleixner wrote:
> On Tue, 10 Jun 2014, Steven Rostedt wrote:
> > On Tue, 10 Jun 2014 20:08:37 +0200 (CEST)
> > Thomas Gleixner <tglx@linutronix.de> wrote:
> > 
> > 
> > > > Perhaps it could simply do ->owner = RT_MUTEX_HAS_WAITERS to make this more
> > > > clear...
> > > 
> > > Good point. The new owner can cleanup the mess.
> > >  
> > 
> > I thought about this too. It should work with the added overhead that
> > every time we go into the unlock slow path, we guarantee that the next
> > lock will go into the lock slowpath.
> > 
> > As long as the new acquired lock does a fast unlock, then we get out of
> > this spiral.
> 
> The alternative solution is to document WHY this is safe. I think I
> prefer that one :)

And actually we keep the waiter bit set in wakeup_next_waiter()
because we only dequeue the waiter from the lock owners pi waiter
list, but not from the lock waiter list.

rt_mutex_set_owner() sets the waiters bit if the lock has waiters. I
agree with Oleg that this is not obvious from the code.

So I add both a comment and open code it.

Thanks,

	tglx