[PROBLEM] hard-lock with kmemtrace, relayfs, and splice

[Pekka Enberg <penberg@cs.helsinki.fi>]

(I'm cc'ing Tom, Jens, and LKML.)

On Fri, 2008-10-10 at 12:10 +0300, Pekka Enberg wrote:
> > I'm seeing a hard lock on my machine when I run kmemtraced with the
> > following patch applied:
> > 
> > http://git.kernel.org/?p=linux/kernel/git/penberg/slab-2.6.git;a=commitdiff;h=17ca1d5506b1db433f0b7167a627bfd55d319dd3
> > 
> > I can enable/disable kmemtrace via the debugfs files fine and can also
> > read the relay files with cat.
> > 
> > Any idea where this is coming from?
> 
> OK, it's the first splice() call in reader_thread() that causes the
> hang. Hmm.

To recap, with a CONFIG_KMEMTRACE enabled kernel from the
"topic/kmemtrace" branch of:

  git://git.kernel.org/pub/scm/linux/kernel/git/penberg/slab-2.6.git topic/kmemtrace

running the "kmemtraced" program from

  git://git.kernel.org/pub/scm/linux/kernel/git/penberg/kmemtrace-user.git

results to a hard lock on my machine. I am unable to find anything
obviously wrong with it and as I can read/write the relay files just
fine, I'm beginning to think it's problem in relayfs splice
implementation.

Tom, thoughts?

		Pekka