Re: [RFC][PATCH 3/5] timekeeping: Avoid possible deadlock from clock_was_set_delayed

[Sasha Levin]

On 12/12/2013 01:59 PM, John Stultz wrote:
> On 12/12/2013 10:32 AM, Sasha Levin wrote:
> > On 12/12/2013 11:34 AM, Sasha Levin wrote:
> > > On 12/11/2013 02:11 PM, John Stultz wrote:
> > > > As part of normal operaions, the hrtimer subsystem frequently calls
> > > > into the timekeeping code, creating a locking order of
> > > >     hrtimer locks -> timekeeping locks
> > > >
> > > > clock_was_set_delayed() was suppoed to allow us to avoid deadlocks
> > > > between the timekeeping the hrtimer subsystem, so that we could
> > > > notify the hrtimer subsytem the time had changed while holding
> > > > the timekeeping locks. This was done by scheduling delayed work
> > > > that would run later once we were out of the timekeeing code.
> > > >
> > > > But unfortunately the lock chains are complex enoguh that in
> > > > scheduling delayed work, we end up eventually trying to grab
> > > > an hrtimer lock.
> > > >
> > > > Sasha Levin noticed this in testing when the new seqlock lockdep
> > > > enablement triggered the following (somewhat abrieviated) message:
> > >
> > > [snip]
> > >
> > > This seems to work for me, I don't see the lockdep spew anymore.
> > >
> > >       Tested-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
> >
> > I think I spoke too soon.
> >
> > It took way more time to reproduce than previously, but I got:
> >
> >
> > -> #1 (&(&pool->lock)->rlock){-.-...}:
> > [ 1195.578519]        [<ffffffff81194803>] validate_chain+0x6c3/0x7b0
> > [ 1195.578519]        [<ffffffff81194d9d>] __lock_acquire+0x4ad/0x580
> > [ 1195.578519]        [<ffffffff81194ff2>] lock_acquire+0x182/0x1d0
> > [ 1195.578519]        [<ffffffff843b0760>] _raw_spin_lock+0x40/0x80
> > [ 1195.578519]        [<ffffffff81153e0e>] __queue_work+0x14e/0x3f0
> > [ 1195.578519]        [<ffffffff81154168>] queue_work_on+0x98/0x120
> > [ 1195.578519]        [<ffffffff81161351>]
> > clock_was_set_delayed+0x21/0x30
> > [ 1195.578519]        [<ffffffff811c4b41>] do_adjtimex+0x111/0x160
> > [ 1195.578519]        [<ffffffff811360e3>] SYSC_adjtimex+0x43/0x80
> > [ 1195.578519]        [<ffffffff8113612e>] SyS_adjtimex+0xe/0x10
> > [ 1195.578519]        [<ffffffff843baed0>] tracesys+0xdd/0xe2
> > [ 1195.578519]
>
> Are you sure you have that patch applied?
>
> With it we shouldn't be calling clock_was_set_delayed() from do_adjtimex().

Hm, It seems that there's a conflict there that wasn't resolved properly. Does this patch
depend on anything else that's not currently in -next?


Thanks,
Sasha

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