linux-kernel - Re: frequent lockups in 3.18rc4

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Date:	Fri, 21 Nov 2014 23:10:42 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Tejun Heo <tj@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Don Zickus <dzickus@...hat.com>, Dave Jones <davej@...hat.com>,
	the arch/x86 maintainers <x86@...nel.org>
Subject: Re: frequent lockups in 3.18rc4

On Fri, Nov 21, 2014 at 08:38:07AM -0800, Andy Lutomirski wrote:
> On Nov 21, 2014 8:27 AM, "Tejun Heo" <tj@...nel.org> wrote:
> >
> > Hello, Andy.
> >
> > On Thu, Nov 20, 2014 at 03:55:09PM -0800, Andy Lutomirski wrote:
> > > That doesn't appear to have anything to with nmi though, right?
> >
> > I thought that was the main offender but, apparently, not any more.
> >
> > > Wouldn't this issue be fixed by moving the vmalloc_fault check into
> > > do_page_fault before exception_enter?
> >
> > Can you please elaborate why that'd fix the issue?  I'm not
> > intimiately familiar with the fault handling so it'd be great if you
> > can give me some pointers in terms of where to look at.
> 
> do_page_fault is called directly from asm.  It does:
> 
>     prev_state = exception_enter();
>     __do_page_fault(regs, error_code, address);
>     exception_exit(prev_state);
> 
> The vmalloc fixup is in __do_page_fault.
> 
> exception_enter does various accounting and tracing things, and I
> think that the recursion in stack trace I saw was in exception_enter.
> 
> If you move the vmalloc fixup before exception_enter() and return if
> the fault was from vmalloc, then you can't recurse.  You need to be
> careful not to touch anything that uses RCU before exception_enter,
> though.

That fixes the exception_enter() recursion but surely more issues with
per cpu memory faults are lurking somewhere now or in the future.

I'm going to add recursion protection to user_exit()/user_enter() anyway.
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