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Date: Sat, 30 Oct 2004 13:28:05 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Lee Revell <rlrevell@joe-job.com>
Cc: Florian Schmidt <mista.tapas@gmx.net>,
       Paul Davis <paul@linuxaudiosystems.com>,
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       Karsten Wiese <annabellesgarden@yahoo.de>,
       jackit-devel <jackit-devel@lists.sourceforge.net>,
       Rui Nuno Capela <rncbc@rncbc.org>
Subject: Re: [Fwd: Re: [patch] Real-Time Preemption, -RT-2.6.9-mm1-V0.4]
Message-ID: <20041030112805.GB23493@elte.hu>
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* Lee Revell <rlrevell@joe-job.com> wrote:

> Ingo, here are some of my traces with the same settings.  These do
> seem to correspond to the xruns.  Many of them look tty related -
> could the recent changes to the tty layer be responsible?  Possibly
> this has nothing to do with RT preempt, but is an unrelated bug in
> -mm?  The xruns do seem to correspond to display activity such as
> switching tabs in gnome-terminal. 
> 
> jackd:1507 userspace BUG: scheduling in user-atomic context!
>  [<c01069fc>] dump_stack+0x1c/0x20 (20)
>  [<c02834d0>] schedule+0x70/0x100 (24)
>  [<c010639b>] work_resched+0x6/0x17 (-8124)

ok, this shows jackd getting preempted by a higher-prio task. This could
be the watchdog thread - could you chrt the watchdog thread to make sure
jackd itself never gets (legitimately) preempted by any other thread?

also, in my tree i fixed this codepath to not trigger an atomicity
warning, since this is an involuntary rescheduling event.

> jackd:1507 userspace BUG: scheduling in user-atomic context!
>  [<c01069fc>] dump_stack+0x1c/0x20 (20)
>  [<c02834d0>] schedule+0x70/0x100 (24)
>  [<c028465d>] down_write_mutex+0xbd/0x180 (36)
>  [<c012cbf6>] __mutex_lock+0x36/0x40 (16)
>  [<c012cc75>] _mutex_lock_irqsave+0x15/0x20 (16)
>  [<c01f1237>] tty_ldisc_try+0x17/0x50 (20)
>  [<c01f1287>] tty_ldisc_ref_wait+0x17/0xc0 (88)
>  [<c01f21fd>] tty_write+0x7d/0x230 (68)
>  [<c01546ac>] vfs_write+0xbc/0x110 (36)
>  [<c01547b1>] sys_write+0x41/0x70 (44)
>  [<c0106367>] syscall_call+0x7/0xb (-8124)

this too i'd consider a false positive, because this particular printout
happened due to a mutex, and a mutex blockage we can consider
involuntary blockage. But ... tty writes can cause BKL locking.

> We know that jackd prints from the realtime thread, and that in theory
> this could be a problem, in practice it works OK.  Maybe some recent
> changes to the tty layer made this problematic.

i'd suggest to do an atomic_off() whenever jackd prints an xrun warning 
from the highprio thread.

> I think there was a patch posted to the JACK mailing list to print
> from a separate thread, I will look into this.

that would be the better longterm solution.

	Ingo
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