Re: fs/dcache.c - BUG: soft lockup - CPU#5 stuck for 22s! [systemd-udevd:1667]

[Al Viro <viro@ZenIV.linux.org.uk>]

On Mon, May 26, 2014 at 11:17:42AM -0700, Linus Torvalds wrote:
> On Mon, May 26, 2014 at 8:27 AM, Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > That's the livelock.  OK.
> 
> Hmm. Is there any reason we don't have some exclusion around
> "check_submounts_and_drop()"?
> 
> That would seem to be the simplest way to avoid any livelock: just
> don't allow concurrent calls (we could make the lock per-filesystem or
> whatever). This whole case should all be for just exceptional cases
> anyway.
> 
> We already sleep in that thing (well, "cond_resched()"), so taking a
> mutex should be fine.

What makes you think that it's another check_submounts_and_drop()?
And not, e.g., shrink_dcache_parent().  Or memory shrinkers.  Or
some twit sitting in a subdirectory and doing stat(2) in a loop, for
that matter...

I really, really wonder WTF is causing that - we have spent 20-odd
seconds spinning while dentries in there were being evicted by
something.  That - on sysfs, where dentry_kill() should be non-blocking
and very fast.  Something very fishy is going on and I'd really like
to understand the use pattern we are seeing there.