Re: kmemleak: Protect the seq start/next/stop sequence by rcu_read_lock()

[Catalin Marinas <catalin.marinas@arm.com>]

On Mon, 2009-08-10 at 20:45 +0200, Ingo Molnar wrote:
> * Catalin Marinas <catalin.marinas@arm.com> wrote:
> 
> > On Sun, 2009-08-02 at 13:14 +0200, Ingo Molnar wrote:
> > > hm, some recent kmemleak patch is causing frequent hard and 
> > > soft lockups in -tip testing (-rc5 based).
> > 
> > Thanks for reporting this. It shouldn't be caused by the patch 
> > mentioned in the subject as this only deals with reading the seq 
> > file which doesn't seem to be the case here.
> 
> Since i turned off kmemleak in -tip completely via the patch below i 
> havent had a single such lockup.
> 
> Have you tried the config i sent - does it work fine for you? For me 
> it locks up on various boxes within a couple of minutes - without 
> doing anything particular beyond building a kernel or so.

I couldn't tried your config as I don't have an x86_64 machine (I only
rely on an x86_32 laptop at home and several ARM machines at work for
testing).

I tried similar config and with the mainline kernel I get some lockups
(several seconds) with CONFIG_PREEMPT disabled on ARM machines or x86
during a scanning episode but it eventually completes the scanning. With
the kmemleak patches for the next merging window, I don't get any
lockups as it has more cond_resched() calls.

Maybe on your x86_64 box you get some bigger objects allocated
(alloc_bootmem, per-cpu, data/bss, NODE_DATA, task stacks) which are
scanned without cond_resched() calls and CONFIG_PREEMPT disabled.
Scanning the memory can even take several minutes especially with
CONFIG_PROVE_LOCKING enabled and maybe that's why you see the lockups.
Enabling CONFIG_PREEMPT reduces the lockup period.

I'll try tomorrow with x86_32 allyesconfig on my laptop and see how it
goes.

Thanks.

-- 
Catalin