linux-kernel - Re: frequent lockups in 3.18rc4

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Date:	Fri, 21 Nov 2014 08:38:07 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Tejun Heo <tj@...nel.org>
Cc:	"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>,
	Frederic Weisbecker <fweisbec@...il.com>,
	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 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.

--Andy

>
> Thanks.
>
> --
> tejun
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