Re: Linux 3.19-rc3

[Sedat Dilek]

On Tue, Jan 6, 2015 at 12:40 PM, Kent Overstreet <kmo@xxxxxxxxxxxxx> wrote:
> 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.

I followed as far as I have understood the subsequent discussion.
Let's see where this will lead to.

I am also very curious about how that aio issue will be fixed.

Thanks Peter and Ken for the vital and hopefully fruitful discussion.

- Sedat -
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