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

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Message-ID: <20140618170038.GQ4669@linux.vnet.ibm.com>
Date:	Wed, 18 Jun 2014 10:00:38 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 Wed, Jun 18, 2014 at 06:43:59PM +0200, Oleg Nesterov wrote:
> On 06/17, Paul E. McKenney wrote:
> >
> > +		if (drop_boost_mutex) {
> > +			rt_mutex_unlock(&rnp->boost_mtx);
> >  			complete(&rnp->boost_completion);
> 
> Well, I still do not understand this ->boost_completion...
> 
> > -	/* Wait until boostee is done accessing mtx before reinitializing. */
> > +	/* Wait for boostee to be done w/boost_mtx before reinitializing. */
> >  	wait_for_completion(&rnp->boost_completion);
> 
> OK, at least we have a comment.
> 
> But let me repeat. Thomas has already fixed rt_mutex, unlock is atomic.
> It doesn't touch this memory after it makes another lock() possible.
> 
> And (contrary to what I said initially) we can rely on this because -rt
> converts spinlock_t into rt_mutex ?

Well, perhaps I should rein in my paranoia on this one.  That said, the
cost of my paranoia is minimal in this slowpath.

							Thanx, Paul

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