Re: [PATCH 2/2]: PCI Error Recovery: Symbios SCSI First Failure

From: Linas Vepstas
Date: Thu Sep 27 2007 - 19:35:24 EST

On Thu, Sep 27, 2007 at 04:10:31PM -0600, Matthew Wilcox wrote:
> In the error handler, we wait_for_completion(io_reset_wait).
> In sym2_io_error_detected, we init_completion(io_reset_wait).
> Isn't it possible that we hit the error handler before we hit the
> io_error_detected path, and thus the completion wait is lost?
> Since the completion is already initialised in sym_attach(), I don't
> think we need to initialise it in sym2_io_error_detected().
> Makes sense to just delete it?

Good catch. But no ... and I had to study this a bit. Bear with me:

It is enough to call init_completion() once, and not once per use:
it initializes spinlocks, which shouldn't be intialized twice.

But, that completion might be used multiple times when there are
multiple errors, and so, before using it a second time, one must
set completion->done = 0. The INIT_COMPLETION() macro does this.

One must have completion->done = 0 before every use, as otherwise,
wait_for_completion() won't actually wait. And since complete_all()
sets x->done += UINT_MAX/2, I'm pretty sure x->done won't be zero
the next time we use it, unless we make it so.

So I need to find a place to safely call INIT_COMPLETION() again,
after the completion has been used. At the moment, I'm stumped
as to where to do this.

---- [think ... think ... think] ----

I think the race you describe above is harmless. The first time
that sym_eh_handler() will run, it will be with SYM_EH_ABORT,
in it doesn't matter if we lose that, since the device is hosed
anyway. At some later time, it will run with SYM_EH_DEVICE_RESET
and then SYM_EH_BUS_RESET and then SYM_EH_HOST_RESET, and we won't
miss those, since, by now, sym2_io_error_detected() will have run.

So, by my reading, I'd say that init_completion() in
sym2_io_error_detected() has to stay (although perhaps
it should be replaced by the INIT_COMPLETION() macro.)
Removing it will prevent correct operation on the second
and subsequent errors.


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