Date: Sun, 27 Jun 1999 18:18:42 +0800 From: Peter Wemm <peter@netplex.com.au> To: Doug Rabson <dfr@nlsystems.com> Cc: Matthew Dillon <dillon@apollo.backplane.com>, current@freebsd.org, mckusick@mckusick.com Subject: Re: BUF_LOCK() related panic.. Message-ID: <19990627101842.3A77B81@overcee.netplex.com.au> In-Reply-To: Your message of "Sun, 27 Jun 1999 10:51:29 %2B0100." <Pine.BSF.4.05.9906271044580.80685-100000@herring.nlsystems.com>
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Doug Rabson wrote:
> On Sun, 27 Jun 1999, Peter Wemm wrote:
>
> > Matthew Dillon wrote:
> > > Ah, yes, some of us were just discussing this in a small mailing list
.
> > > Hopefully Kirk will pick up on it soon. Ah well.. someone else gets
to b
> > e
> > > the brunt of it for a change :-). Kirk doesn't have an SMP box so he
> > > didn't see the bug.
> > >
> > > I have tentitively tracked the problem down to the apparent inability
of
> > > lockmgr() locks to function from interrupts, even when used in a
> > > non-blocking manner, due to the simplelock's it uses internally. The
> > > new buffer cache code Kirk committed switched from B_BUSY (manually
> > > implemented locks) to lockmgr() locks. I think what is going on is
> > > that mainline code is getting a simplelock and then an interrupt is
> > > coming along and also trying to get the same lock, but I can't be sur
e
> > > because my DDB backtraces are somewhat munged.
> >
> > In this case, it was just a programming error.. The key to remember is tha
t
> > the simplelocks are used to protect the state of the complex lock, they are
> > not the lock themselves. lockmgr() holds the interlock while gaining or
> > removing references etc and then frees the simplelock so that it can sleep
> > if required etc. The actual implementation of the simplelock routines
> > is interrupt safe (and has to be).
>
> The simple_lock* macros don't seem to use the interrupt safe versions
> (ss_lock etc). What happens if an interrupt is recieved after gaining
> buftimelock and the interrupt routine also tries to call BUF_LOCK?
Good question, but I'm not sure ss_lock is what's needed either since that
does a cli for the duration of the simplelock being held..
I think the BUF_*() inlines need internal splbio() protection since
a biodone() can be called from the tail end of an interrupt, and that *does*
try and get a simplelock during a BUF_UNLOCK()... (and BUF_REFCNT()).
Cheers,
-Peter
--
Peter Wemm - peter@FreeBSD.org; peter@yahoo-inc.com; peter@netplex.com.au
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