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Date: Sat, 28 Sep 2002 19:38:06 +0100
From: John Levon <movement@marcelothewonderpenguin.com>
To: linux-kernel@vger.kernel.org
Subject: Re: Sleeping function called from illegal context...
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On Sat, Sep 28, 2002 at 02:27:44PM -0400, Robert Love wrote:

> > NMI interrupt handler cannot block so it trylocks against a spinlock
> > instead. Buffer processing code needs to block against concurrent NMI
> > interrupts so takes the spinlock for them. All actual blocks on the
> > spinlock are beneath a down() on another semaphore, so a sleep whilst
> > holding the spinlock won't actually cause deadlock.
> 
> If all accesses to the spinlock are taken under a semaphore, then the
> spinlock is not needed (i.e. the down'ed semaphore provides the same
> protection), or am I missing something?
> 
> If this is not the case - e.g. there are other accesses to these locks -
> then you cannot sleep, no?

The other accessors are spin_trylock()ers, as I mentioned. They will not
block but they are not under the semaphore.

The spinlock cannot be a semaphore because NMI interrupts do not take to
kindly to up()

regards
john

-- 
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