Subject: Re: Mutex question
To: Steven M. Bellovin <email@example.com>
From: Eric Haszlakiewicz <firstname.lastname@example.org>
Date: 03/16/2006 11:28:04
On Thu, Mar 16, 2006 at 11:49:47AM -0500, Steven M. Bellovin wrote:
> On Thu, 16 Mar 2006 09:16:57 -0500 (EST), Matthew Orgass
> <email@example.com> wrote:
> > On 2006-03-16 firstname.lastname@example.org wrote:
> > > If I want to aquire a mutex from:
> > >
> > > 1) Interrupt handler
> > > 2) Callout (timeout)
> > >
> > > How do I do that? Do I have to use a soft interrupt handler?
> > No, you just need to use the right spl calls to block the hard interrupt
> > in the callout (placed outside the lock).
> The question is this: what do you do in the interrupt handler or
> timeout if you can't get the mutex? You can't sleep; there's no
> process context.
I think you have to spin. Since the splfoo() in non-interrupt code
ensures that the current processor can't have been in the middle of the
mutex protected code, you know that it must be some other processor that
holds the lock. Eventually, that processor will release the mutex and
the interrupt handler will get it. Of course, if the non-interrupt code
holds that mutex for long the interrupt latency will suck.
I don't know what you'd do in the callout. Maybe re-register the
callout and try again later?