Message-ID: <1371078380.20762.29.camel@hp>
Subject: Re: [PATCH] spin_unlock*_no_resched()
From: Kirill Tkhai <tkhai@yandex.ru>
Reply-To: tkhai@yandex.ru
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>, tglx@linutronix.de
Date: Thu, 13 Jun 2013 03:06:20 +0400
In-Reply-To: <1371042443.9844.255.camel@gandalf.local.home>
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	 <20130612121532.GD3204@twins.programming.kicks-ass.net>
	 <1371042443.9844.255.camel@gandalf.local.home>
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On Wed, 2013-06-12 at 09:07 -0400, Steven Rostedt wrote:
> 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.

Steven thanks for your explanation and Peter's, now I looked to this
from another side.

If we speak about combined primitive does it have to be a special
variant of schedule_spin_unlock_* for every irq state? The simplest way
is to do local_irq_enable() always before schedule() call, but I'm not
sure that this is good for all platforms.

For -rt everything of this is completely useless, because number of
raw_spin_locks is small. Maybe changes for some another types of locks
will applicable.

Kirill

> 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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