netdev - Re: Strange soft lockup detected message (looks like spin

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Date:	Fri, 4 May 2007 15:02:36 -0400
From:	lsorense@...lub.uwaterloo.ca (Lennart Sorensen)
To:	Don Fry <pcnet32@...izon.net>
Cc:	Frederik Deweerdt <deweerdt@...e.fr>, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: Strange soft lockup detected message (looks like spin_lock bug in pcnet32)

On Fri, May 04, 2007 at 11:24:33AM -0700, Don Fry wrote:
> All instances of obtaining the lock in pcnet32 are done as
> spin_lock_irqsave except the interrupt handler itself.  The interrupt mask
> needs to be saved everywhere else, but the interrupt handler is known not
> to need to save the flags.
> 
> If the lock is held and the same CPU tries to get the lock again, it will
> wait a very long time ;-(.  I believe the locking is fine for a
> non-preemptable kernel, but I have little experience with a preemptable
> kernel.
> 
> When does a preemptable kernel allow interrupts to occur?

I have no idea actually.

> Is there a bug in this particular architectures locking code?

On i386?  I hope not.

> From looking at preempt-locking.txt the driver has (1) no per-cpu data,
> (2) 'CPU state protection' should be fine,
> (3) the 'lock is acquired and released by the same task'.
> I don't see a problem unless I am misunderstanding something.

Well I don't know, but something is going wrong and causing the soft
lock up.  I must admit I am surprised if an interrupt can occour while
handling an interrupt, but then again maybe that is supposed to be
allowed.

--
Len Sorensen
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