linux-kernel - Re: [PATCH] spin_unlock*_no

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Message-ID: <1371042443.9844.255.camel@gandalf.local.home>
Date:	Wed, 12 Jun 2013 09:07:23 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Kirill Tkhai <tkhai@...dex.ru>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>, tglx@...utronix.de
Subject: Re: [PATCH] spin_unlock*_no_resched()

On Wed, 2013-06-12 at 14:15 +0200, Peter Zijlstra wrote:

> So I absolutely hate this API because people can (and invariably will)
> abuse it; much like they did/do preempt_enable_no_resched().

Me too.

> 
> IIRC Thomas even maps preempt_enable_no_resched() to preempt_enable() in
> -rt to make sure we don't miss preemption points due to stupidity.
> 
> He converted the 'few' sane sites to use schedule_preempt_disabled(). In
> that vein, does it make sense to introduce schedule_spin_locked()?
> 

I was thinking the exact same thing when I read this patch. This is a
strict policy that we should enforce and not let individual developers
implement. Yes, a schedule_spin_unlock() would work nicely. The API will
enforce the two to be used together. Otherwise, I can envision seeing
things like:

	preempt_disable();
	[...]

	spin_lock(x);

	spin_unlock_no_resched(x);

	[...]

	preempt_enable();

And developers having no idea why the above is broken. Although, I would
say the above is broken for other reasons, but I was just using that to
show the craziness such an API would give to us.

-- Steve



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