Re: 3.14-rc2 XFS backtrace because irqs_disabled.

[Dave Chinner <david@fromorbit.com>]

On Wed, Feb 12, 2014 at 05:10:38PM +1100, Dave Chinner wrote:
> On Wed, Feb 12, 2014 at 12:50:27AM -0500, Dave Jones wrote:
> > On Wed, Feb 12, 2014 at 04:40:43PM +1100, Dave Chinner wrote:
> > 
> >  > None of the XFS code disables interrupts in that path, not does is
> >  > call outside XFS except to dispatch IO. The stack is pretty deep at
> >  > this point and I know that the standard (non stacked) IO stack can
> >  > consume >3kb of stack space when it gets down to having to do memory
> >  > reclaim during GFP_NOIO allocation at the lowest level of SCSI
> >  > drivers. Stack overruns typically show up with symptoms like we are
> >  > seeing.
> >  > ..
> >  > 
> >  > Dave, before chasing ghosts, can you (like Eric originally asked)
> >  > turn on stack overrun detection?
> > 
> > CONFIG_DEBUG_STACKOVERFLOW ? Already turned on.
> 
> That only checks stack usage when an interrupt is taken. If no
> interrupts are taken when stack usage is within 128 bytes of
> overflow, then it doesn't catch it.
> 
> I tend to use CONFIG_DEBUG_STACK_USAGE=y as it records the maximum
> stack usage of a process via canary overwrites and it records it in
> do_exit(). I also use the stack tracer to record the largest stack
> usage seen so I know exactly what code paths are approaching stack
> overruns...

FYI, just creating lots of files with open(O_CREAT):

[  348.718357] fs_mark (4828) used greatest stack depth: 2968 bytes left
[  348.769846] fs_mark (4814) used greatest stack depth: 2312 bytes left
[  349.777717] fs_mark (4826) used greatest stack depth: 2280 bytes left
[  418.139415] fs_mark (4928) used greatest stack depth: 1936 bytes left
[  460.492282] fs_mark (4993) used greatest stack depth: 1336 bytes left
[  544.825418] fs_mark (5104) used greatest stack depth: 1112 bytes left
[  689.503970] fs_mark (5265) used greatest stack depth: 1000 bytes left

We've got absolutely no spare stack space anymore in the IO path.
And the IO path can't get much simpler than filesystem -> virtio
block device.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com