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

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Message-ID: <alpine.DEB.2.10.1406102210580.5170@nanos>
Date:	Tue, 10 Jun 2014 22:13:20 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Clark Williams <williams@...hat.com>
Subject: Re: safety of *mutex_unlock() (Was: [BUG] signal: sighand unprotected
 when accessed by /proc)

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@...utronix.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
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