Date: Tue, 4 Jan 2005 10:33:30 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Christoph Hellwig <hch@infradead.org>, Andrew Morton <akpm@osdl.org>,
       linux-kernel@vger.kernel.org
Subject: Re: 2.6.10-mm1
Message-ID: <20050104093330.GA13602@elte.hu>
References: <20050103011113.6f6c8f44.akpm@osdl.org> <20050103115120.GB18408@infradead.org> <20050104090408.GA12197@elte.hu> <20050104092612.GA2371@infradead.org>
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* Christoph Hellwig <hch@infradead.org> wrote:

> On Tue, Jan 04, 2005 at 10:04:08AM +0100, Ingo Molnar wrote:
> > or is it the addition of _smp_processor_id() as a way to signal 'this
> > smp_processor_id() call in a preemptible region is fine, trust me'?
> 
> Yes.
> 
> > We
> > could do smp_processor_id_preempt() or some other name - any better
> > suggestions?
> 
> I'd just kill the debug check and rely on the eye of the review to not
> let new users of smp_processor_id slip in.

relying on that is quite futile. E.g. in the block IO code it needed 3-4
iterations even after the first instance was found to get all the cases
right. There are functions that are always called from under a lock then
some unlocked call happens and we've got trouble. Often the bug is some
very rare and obscure corruption of a statistics value, nobody really
notices that.

by today i think we've identified most of the places that can safely do
smp_processor_id() in a preemptible section (in x86 and x64) - it's only
around 3% of the total smp_processor_id() use. I'd rather allow these
exceptions and flag new exceptions as they get added - they are added at
least an order of magnitude more rarely than smp_processor_id() gets
added.

	Ingo
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