linux-kernel - Re: [PATCH [RT] 11/14] optimize the !printk fastpath through the lock acquisition

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Date:	Mon, 25 Feb 2008 00:20:14 -0500
From:	Gregory Haskins <gregory.haskins.ml@...il.com>
To:	"Bill Huey (hui)" <bill.huey@...il.com>
CC:	gregory.haskins@...il.com, Pavel Machek <pavel@....cz>,
	Gregory Haskins <ghaskins@...ell.com>, mingo@...e.hu,
	a.p.zijlstra@...llo.nl, tglx@...utronix.de, rostedt@...dmis.org,
	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	kevin@...man.org, cminyard@...sta.com, dsingleton@...sta.com,
	dwalker@...sta.com, npiggin@...e.de, dsaxena@...xity.net,
	ak@...e.de, gregkh@...e.de, sdietrich@...ell.com,
	pmorreale@...ell.com, mkohari@...ell.com
Subject: Re: [PATCH [RT] 11/14] optimize the !printk fastpath through the
 lock acquisition

Bill Huey (hui) wrote:
> 
> The might_sleep is annotation and well as a conditional preemption
> point for the regular kernel. You might want to do a schedule check
> there, but it's the wrong function if memory serves me correctly. It's
> reserved for things that actually are design to sleep.

Note that might_sleep() already does a cond_resched() on the 
configurations that need it, so I am not sure what you are getting at 
here.  Is that not enough?

> The rt_spin*()
> function are really a method of preserving BKL semantics across real
> schedule() calls. You'd have to use something else instead for that
> purpose like cond_reschedule() instead.

I dont quite understand this part either.  From my perspective, 
rt_spin*() functions are locking constructs that might sleep (or might 
spin with the new patches), and they happen to be BKL and wakeup 
transparent.  To me, either the might_sleep() is correct for all paths 
that don't fit the in_atomic-printk exception, or none of them are.

Are you saying that the modified logic that I introduced is broken?  Or 
that the original use of the might_sleep() annotation inside this 
function is broken?

-Greg
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