linux-kernel - Re: [PATCH tip/core/rcu 01/11] mm: Place preemption point in do

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Message-ID: <20130925201820.GA9093@linux.vnet.ibm.com>
Date:	Wed, 25 Sep 2013 13:18:20 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	mathieu.desnoyers@...icios.com, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	darren@...art.com, fweisbec@...il.com, sbw@....edu,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Michel Lespinasse <walken@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH tip/core/rcu 01/11] mm: Place preemption point in
 do_mlockall() loop

On Wed, Sep 25, 2013 at 12:35:37PM -0700, Andrew Morton wrote:
> On Wed, 25 Sep 2013 06:48:04 -0700 "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > On Tue, Sep 24, 2013 at 09:10:47PM -0700, Andrew Morton wrote:
> > > On Tue, 24 Sep 2013 18:29:11 -0700 "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> > > 
> > > > --- a/mm/mlock.c
> > > > +++ b/mm/mlock.c
> > > > @@ -736,6 +736,7 @@ static int do_mlockall(int flags)
> > > >  
> > > >  		/* Ignore errors */
> > > >  		mlock_fixup(vma, &prev, vma->vm_start, vma->vm_end, newflags);
> > > > +		cond_resched();
> > > >  	}
> > > >  out:
> > > >  	return 0;
> > > 
> > > Might need one in munlock_vma_pages_range() as well - it's a matter of
> > > finding the right test case.  This will be neverending :(
> > 
> > Indeed...  I suspect that Trinity running on big-memory systems will
> > eventually find most of them via RCU CPU stall warnings, but as you say...
> > 
> > Would you like the corresponding change to munlock_vma_pages_range()
> > beforehand?
> 
> Can't decide.  If we went and poked holes in every place which looks
> like it loops for a long time, we'd be poking holes everywhere, some of
> them unnecessary.  otoh if we wait around for people to say "hey" then
> it will take a very long time to poke all the needed holes.

Yep.

> The best approach would be for someone to sit down, identify all the
> potential problem spots, attempt to craft a userspace exploit to verify
> that each one really is a problem, then fix it.  Nobody will bother
> doing this.

And if someone does bother doing this, there will no doubt be some debate
about whether or not the exploit is reasonable.

> So I dunno.  Stop asking difficult questions ;)

;-) ;-) ;-)

							Thanx, Paul

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