On Sun, 27 Jun 1999, Peter Wemm wrote:
> 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()).
In the long term, we probably need an spl-aware simplelock or maybe the
cunning no-cost interrupt thread scheme which BSDi are using.
--
Doug Rabson Mail: [EMAIL PROTECTED]
Nonlinear Systems Ltd. Phone: +44 181 442 9037
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