linux-kernel - Re: [BUG] Lockless patches cause hardlock under heavy IO

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Message-ID: <20080624153238.GD7978@linux.vnet.ibm.com>
Date:	Tue, 24 Jun 2008 08:32:38 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Ryan Hope <rmh3093@...il.com>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-mm@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] Lockless patches cause hardlock under heavy IO

On Tue, Jun 24, 2008 at 11:12:03AM -0400, Ryan Hope wrote:
> Well i tried to run pure -mm this weekend, it locked as soon as I got
> into gnome so I applied a couple of the bug fixes from lkml and -mm
> seems to be running stable now. I cant seem to get it to hard lock
> now, at least not doing the simple stuff that was causing it to hard
> lock on my other patchset, either the lockless patches expose some bug
> that in -rc6 or lockless requires some other patches further up in the
> -mm series file.

Cool!!!  Any guess as to which of the bug fixes did the trick?
Failing that, a list of the bug fixes that you applied?

							Thanx, Paul

> On Mon, Jun 23, 2008 at 8:13 PM, Nick Piggin <nickpiggin@...oo.com.au> wrote:
> > On Monday 23 June 2008 23:05, Paul E. McKenney wrote:
> >> On Mon, Jun 23, 2008 at 09:54:52PM +1000, Nick Piggin wrote:
> >> > On Monday 23 June 2008 13:51, Ryan Hope wrote:
> >> > > well i get the hardlock on -mm with out using reiser4, i am pretty
> >> > > sure is swap related
> >> >
> >> > The guys seeing hangs don't use PREEMPT_RCU, do they?
> >> >
> >> > In my swapping tests, I found -mm3 to be stable with classic RCU, but
> >> > on a hunch, I tried PREEMPT_RCU and it crashed a couple of times rather
> >> > quickly. First crash was in find_get_pages so I suspected lockless
> >> > pagecache doing something subtly wrong with the RCU API, but I just got
> >> > another crash in __d_lookup:
> >>
> >> Could you please send me a repeat-by?  (At least Alexey is no longer
> >> alone!)
> >
> > OK, I had DEBUG_PAGEALLOC in the .config, which I think is probably
> > important to reproduce it (but the fact that I'm reproducing oopses
> > with << PAGE_SIZE objects like dentries and radix tree nodes indicates
> > that there is even more free-before-grace activity going undetected --
> > if you construct a test case using full pages, it might become even
> > easier to detect with DEBUG_PAGEALLOC).
> >
> > 2 socket, 8 core x86 system.
> >
> > I mounted two tmpfs filesystems, one contains a single large file
> > which is formatted as 1K block size ext3 and mounted loopback, the
> > other is used directly. Linux kernel source is unpacked on each mount
> > and concurrent make -j128 on each. This pushes it pretty hard into
> > swap. Classic RCU survived another 5 hours of this last night.
> >
> > But that's a fairly convoluted test for an RCU problem. I expect it
> > should be easier to trigger with something more targetted...
> >
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