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

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Date:	Tue, 10 Jun 2014 22:05:20 +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, 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 :)

Thanks,

	tglx


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