Re: task imap:2958 blocked for more than 120 seconds

From: Chris Mason
Date: Wed Jan 13 2010 - 20:07:48 EST

On Wed, Jan 13, 2010 at 11:37:33PM +0100, Johannes Hirte wrote:
> Am Sonntag 10 Januar 2010 21:19:26 schrieb Chris Mason:
> > On Sun, Jan 10, 2010 at 09:05:46PM +0100, Johannes Hirte wrote:
> > > I've observed this hanging task now several times. Not sure when this
> > > started, but 2.6.32 is affected too, IIRC. I don't have a test pattern
> > > for this. Dovecot imap triggers this from time to time. I've enabled
> >
> > > and got this two tasks which hang:
> > You're stuck on a read, could you please do a sysrq-w when this happens?
> Will do so when it happens again.

No need, I reread the trace, you're stuck in copy_from_user and probably
hitting a deadlock in the btrfs page faulting path. This is pretty
rare, but I'll put it into the queue to fix up.
> > Also, do you eventually recover or are you stuck forever?
> I didn't wait too long when it happened, so I'm not sure. The longest time
> I've waited was 20-30min, until reboot without recover. So either it's stuck
> forever or it takes really long to recover.
> And one question I have: How do you identify a read in this call trace?

Lock page followed by sync page is usually for reading.


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