Message-ID: <43F4D8AB.5040708@microgate.com>
Date: Thu, 16 Feb 2006 13:55:23 -0600
From: Paul Fulghum <paulkf@microgate.com>
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To: snits@snitselaar.org
CC: linux-kernel@vger.kernel.org
Subject: Re: Problem: Possible deadlock for 2.4 SMP systems
References: <48822.198.115.32.5.1139956559.squirrel@cantor.snitselaar.org>
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Gerard Snitselaar wrote:
> What appears to happen is cpu0 calls cli() in
> shutdown() (drivers/char/serial.c), grabbing global_irq_lock.
> Meanwhile cpu1 sets IRQ_INPROGRESS, and eventually calls
> handle_IRQ_event() and spins on global_irq_lock in irq_enter().
> CPU0 calls free_irq() and eventually gets to the point where
> it spins while IRQ_INPROGRESS is set. Since cpu0 is holding
> global_irq_lock, cpu1 can't do its work and clear IRQ_INPROGRESS.

 From looking at irq.c (2.4.31) I guess that calling free_irq()
on SMP after cli() is not safe because of the race you describe.

> I read somewhere that global_irq_lock is deprecated, so is there
> something that the serial driver should be doing instead of cli()
> and restore_flags() in shutdown()?

shutdown() seems a little backwards:
it calls free_irq(), then it disables device interrupts.

One way of handling this may be to move the code
block (the if statement after 'Free the IRQ' comment)
that calls free_irq() to after the restore_flags().

At that point, the device is no longer generating
interrupts and has been removed from the IRQ_ports
list so the ISR will not touch the device instance
and free_irq() can finish safely.

-- 
Paul Fulghum
Microgate Systems, Ltd.
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