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Message-ID: <1361558250.14402.6.camel@thor.lan>
Subject: Re: [PATCH v4 00/32] ldisc patchset
From: Peter Hurley <peter@hurleysoftware.com>
To: Sasha Levin <levinsasha928@gmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jslaby@suse.cz>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org,
        Ilya Zykov <ilya@ilyx.ru>, Dave Jones <davej@redhat.com>,
        Michael Ellerman <michael@ellerman.id.au>,
        Shawn Guo <shawn.guo@linaro.org>
Date: Fri, 22 Feb 2013 13:37:30 -0500
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On Thu, 2013-02-21 at 08:38 -0500, Peter Hurley wrote:
> On Thu, 2013-02-21 at 08:16 -0500, Sasha Levin wrote:
> > On 02/20/2013 03:02 PM, Peter Hurley wrote:
> > > Sasha and Dave, my trinity testbeds die in other areas right now;
> > > I would really appreciate if you would please re-test this series.
> > 
> > Hi Peter,
> > 
> > I saw this twice in overnight fuzzing:
> > 
> > [ 1473.912280] =================================
> > [ 1473.913180] [ BUG: bad contention detected! ]
> > [ 1473.914071] 3.8.0-next-20130220-sasha-00038-g1ad55df-dirty #8 Tainted: G        W
> > [ 1473.915684] ---------------------------------
> > [ 1473.916549] kworker/1:1/361 is trying to contend lock (&tty->ldisc_sem) at:
> > [ 1473.918031] [<ffffffff81c493df>] tty_ldisc_ref+0x1f/0x60
> > [ 1473.919060] but there are no locks held!
> 
> Ahh, of course. That explains why the rwsem trylock doesn't track lock
> stats -- because by the time lock_contended() is called, up_write()
> could have just called lockdep_release(), so that it appears as if the
> lock has been released when in fact it has not but is about to.
> 
> I'll just remove the lock contention test from the trylocks.

Hi Sasha,

Sorry for the delay. I was actually looking into if I could tickle
lockdep into just recording the lock contention without testing, but
unfortunately, changes to where lockdep stores the contention now
requires the lockdep state to have an existing owner.

So here's the trivial patch:

--- >% ---
Subject: [PATCH] tty: Drop lock contention stat from ldsem trylocks

When lockdep is notified of lock contention, lockdep expects its
internal state to indicate the lock is held. Since trylocks cannot
reflect this state unless the lock is actually acquired, contention
stats cannot be collected for trylocks.

Fixes:
[ 1473.912280] =================================
[ 1473.913180] [ BUG: bad contention detected! ]
[ 1473.914071] 3.8.0-next-20130220-sasha-00038-g1ad55df-dirty #8 Tainted: G        W
[ 1473.915684] ---------------------------------
[ 1473.916549] kworker/1:1/361 is trying to contend lock (&tty->ldisc_sem) at:
[ 1473.918031] [<ffffffff81c493df>] tty_ldisc_ref+0x1f/0x60
[ 1473.919060] but there are no locks held!
[ 1473.919813]
[ 1473.919813] other info that might help us debug this:
[ 1473.920044] 2 locks held by kworker/1:1/361:
[ 1473.920044]  #0:  (events){.+.+.+}, at: [<ffffffff811328a8>] process_one_work+0x228/0x6a0
[ 1473.920044]  #1:  ((&buf->work)){+.+...}, at: [<ffffffff811328a8>] process_one_work+0x228/0x6a0
[ 1473.920044]
[ 1473.920044] stack backtrace:
[ 1473.920044] Pid: 361, comm: kworker/1:1 Tainted: G        W    3.8.0-next-20130220-sasha-00038-g1ad55df-dirty #8
[ 1473.920044] Call Trace:
[ 1473.920044]  [<ffffffff81c493df>] ? tty_ldisc_ref+0x1f/0x60
[ 1473.920044]  [<ffffffff81182026>] print_lock_contention_bug+0xf6/0x110
[ 1473.920044]  [<ffffffff81184973>] lock_contended+0x213/0x4e0
[ 1473.920044]  [<ffffffff81c4bb41>] ldsem_down_read_trylock+0xb1/0xc0
[ 1473.920044]  [<ffffffff81c493df>] tty_ldisc_ref+0x1f/0x60
[ 1473.920044]  [<ffffffff81c4a687>] flush_to_ldisc+0x37/0x1a0
[ 1473.920044]  [<ffffffff811329e6>] process_one_work+0x366/0x6a0
[ 1473.920044]  [<ffffffff811328a8>] ? process_one_work+0x228/0x6a0
[ 1473.920044]  [<ffffffff811332a8>] worker_thread+0x238/0x370
[ 1473.920044]  [<ffffffff81133070>] ? rescuer_thread+0x310/0x310
[ 1473.920044]  [<ffffffff8113d873>] kthread+0xe3/0xf0
[ 1473.920044]  [<ffffffff8113d790>] ? flush_kthread_work+0x1f0/0x1f0
[ 1473.920044]  [<ffffffff83dab4fc>] ret_from_fork+0x7c/0xb0
[ 1473.920044]  [<ffffffff8113d790>] ? flush_kthread_work+0x1f0/0x1f0

Reported-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
---
 drivers/tty/tty_ldsem.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/drivers/tty/tty_ldsem.c b/drivers/tty/tty_ldsem.c
index 0ab5b09..ce26fc0 100644
--- a/drivers/tty/tty_ldsem.c
+++ b/drivers/tty/tty_ldsem.c
@@ -419,7 +419,6 @@ int ldsem_down_read_trylock(struct ld_semaphore *sem)
 			return 1;
 		}
 	}
-	lock_stat(sem, contended);
 	return 0;
 }
 
@@ -455,7 +454,6 @@ int ldsem_down_write_trylock(struct ld_semaphore *sem)
 	}
 
 	raw_spin_unlock_irq(&sem->wait_lock);
-	lock_stat(sem, contended);
 	return 0;
 }
 
-- 
1.8.1.2