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

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Message-Id: <20130925123537.c948095dd10fbf6e1b8708f4@linux-foundation.org>
Date:	Wed, 25 Sep 2013 12:35:37 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	paulmck@...ux.vnet.ibm.com
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, 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.

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.

So I dunno.  Stop asking difficult questions ;)
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