Re: Linux 3.19-rc3

[Kent Overstreet]

On Tue, Jan 06, 2015 at 12:25:39PM +0100, Sedat Dilek wrote:
> On Tue, Jan 6, 2015 at 12:07 PM, Kent Overstreet <kmo@xxxxxxxxxxxxx> wrote:
> > On Tue, Jan 06, 2015 at 12:01:12PM +0100, Peter Zijlstra wrote:
> >> On Tue, Jan 06, 2015 at 11:18:04AM +0100, Sedat Dilek wrote:
> >> > On Tue, Jan 6, 2015 at 11:06 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >> > > On Tue, Jan 06, 2015 at 10:57:19AM +0100, Sedat Dilek wrote:
> >> > >> [   88.028739]  [<ffffffff8124433f>] aio_read_events+0x4f/0x2d0
> >> > >>
> >> > >
> >> > > Ah, that one. Chris Mason and Kent Overstreet were looking at that one.
> >> > > I'm not touching the AIO code either ;-)
> >> >
> >> > I know, I was so excited when I see nearly the same output.
> >> >
> >> > Can you tell me why people see "similiar" problems in different areas?
> >>
> >> Because the debug check is new :-) It's a pattern that should not be
> >> used but mostly works most of the times.
> >>
> >> > [  181.397024] WARNING: CPU: 0 PID: 2872 at kernel/sched/core.c:7303
> >> > __might_sleep+0xbd/0xd0()
> >> > [  181.397028] do not call blocking ops when !TASK_RUNNING; state=1
> >> > set at [<ffffffff810b83bd>] prepare_to_wait_event+0x5d/0x110
> >> >
> >> > With similiar buzzwords... namely...
> >> >
> >> > mutex_lock_nested
> >> > prepare_to_wait(_event)
> >> > __might_sleep
> >> >
> >> > I am asking myself... Where is the real root cause - in sched/core?
> >> > Fix one single place VS. fix the impact at several other places?
> >>
> >> No, the root cause is nesting sleep primitives, this is not fixable in
> >> the one place, both prepare_to_wait and mutex_lock are using
> >> task_struct::state, they have to, no way around it.
> >
> > No, it's completely possible to construct a prepare_to_wait() that doesn't
> > require messing with the task state. Had it for years.
> >
> > http://evilpiepirate.org/git/linux-bcache.git/log/?h=aio_ring_fix
> 
> I am just rebuilding a new kernel with "aio_ring_fix" included - I
> have tested this alread with loop-mq and it made the call-trace in aio
> go away.
> 
> 
> Jut curious...
> How would a patch look like a patch to fix the sched-fanotify issue
> with a conversion to "closure waitlist"?

wait_queue_head_t	-> struct closure_waitlist
DEFINE_WAIT()		-> struct closure cl; closure_init_stack(&cl)
prepare_to_wait()	-> closure_wait(&waitlist, &cl)
schedule()		-> closure_sync()
finish_wait()		-> closure_wake_up(); closure_sync()

That's the standard conversion, I haven't looked at the fanotify code before
just now but from a cursory glance it appears that all should work here. Only
annoying thing is the waitqueue here is actually part of the poll interface (if
I'm reading this correctly), so I dunno what I'd do about that.

Also FYI: closure waitlists are currently singly linked, thus there's no direct
equivalent to finish_wait(), the conversion I gave works but will lead to
spurious wakeups. I kinda figured I was going to have to switch to doubly linked
lists eventually though.
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