linux-kernel - Re: [PATCH 11/11] s390: convert to generic helpers for IPI function calls

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Date:	Wed, 23 Apr 2008 13:47:44 +0200
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	npiggin@...e.de, torvalds@...ux-foundation.org,
	peterz@...radead.org, sam@...nborg.org,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH 11/11] s390: convert to generic helpers for IPI
	function calls

> > > Your generic patch changes semantics of smp_call_function(). At least the
> > > s390 version of smp_call_function() mask used to not return unless all IPIs
> > > where received on the target cpus.
> > > Your patch changes that. It will already return as soon as all IPIs have
> > > been sent. Unfortunately Martin's etr code in arch/s390/kernel/time.c
> > 
> > Yep, this is what I mentioned in the intro mail.
> > 
> > > relies on exactly the old semantics.
> > 
> > OK, I could not see anything which relied on that and I did think that
> > it was a somewhat odd interface. By the time that smp_call_function()
> > stops waiting for startups, we could easily be in the condition that the
> > N-1 CPUs executed the function part a long time ago. I just didn't see
> > much usefulness in that, you may as well just set wait == 1 in that
> > case.
> > 
> > > Since this seems to be an s390 only issue, could you just drop the s390
> > > conversion patch? We'll have to think about how to change the err code
> > > and convert later I guess.
> > 
> > I'll drop the s390 bits. I can easily add something like
> > smp_call_function_foo() that has this behaviour.
> 
> I added an smp_call_function_sync() for this purpose and kept the s390
> conversion, I would very much appreciate if you could look it over. I
> wont post the full patchset again today, but you can inspect it here:
> 
> http://git.kernel.dk/?p=linux-2.6-block.git;a=shortlog;h=refs/heads/generic-ipi
> 
> The interesting bits are patch #1 for the generic bits and patch #11 for
> the s390 part. Thanks!

I don't think that works: the old code also relied on the fact that there
could be only one cpu sending an smp_call_* IPI (serialized by call_lock).
The current etr code exploits that to serialize all cpus:

Do smp_call_function() to force all other cpus into clock_sync_cpu_start()
where they sort of busy wait(!). Then the master cpu is syncing the clock
and when it finished the other cpus may continue again and finally leave
the smp_call_function interrupt handler.

Your patch series however doesn't make sure anymore that there is only
one cpu doing an smp_call_function*, so we may deadlock as soon as two
cpus are trying to synchronize all cpus this way.
After all I think the etr code should be converted to use stop_machine_run
instead. However that doesn't allow for master/slave cpus.
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