From mboxrd@z Thu Jan  1 00:00:00 1970
Date: Wed, 17 Nov 2010 08:45:00 +0200
From: Lucio De Re <lucio@proxima.alt.za>
To: Fans of the OS Plan 9 from Bell Labs <9fans@9fans.net>
Message-ID: <20101117064500.GA3597@fangle.proxima.alt.za>
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	<4ee4b7c2dd207e953599fc618df2c456@gmx.de>
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Subject: Re: [9fans] That deadlock, again
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On Wed, Nov 17, 2010 at 06:22:33AM +0100, cinap_lenrek@gmx.de wrote:
>
> qpc is the just the caller of the last successfull *acquired* qlock.
> what we know is that the exportfs proc spins in the q->use taslock
> called by qlock() right?  this already seems wired...  q->use is held
> just long enougth to test q->locked and manipulate the queue.  also
> sched() will avoid switching to another proc while we are holding tas
> locks.
>
I think I'm with you, probably not quite to the same depth of understanding.

> i would like to know which qlock is the kernel is trying to acquire
> on behalf of exportfs that is also reachable from the etherread4
> code.
>
... and from whatever the other proc is that also contributes to this
jam. I don't have the name right in front of me, but I will post it
separately.  As far as I know it's always those two that interfere with
exportfs and usually together, only a short time apart.

> one could move:
>
> 	up->qpc = getcallerpc(&q);
>
> from qlock() before the lock(&q->use); so we can see from where that
> qlock gets called that hangs the exportfs call, or add another magic
> debug pointer (qpctry) to the proc stucture and print it in dumpaproc().
>
I think I'll do the latter; even though it's more complex, it can be a
useful debugging tool in future.  I wouldn't leave in the kernel code,
but it would be worth being able to refer to it when the occasion arises.

How do you expect this qpctry to be initialised/set?

++L