linux-kernel - Re: [PATCH RFC] pm_qos

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Message-Id: <1218736137.10800.234.camel@twins>
Date:	Thu, 14 Aug 2008 19:48:57 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	mgross@...ux.intel.com
Cc:	John Kacur <jkacur@...il.com>, LKML <linux-kernel@...r.kernel.org>,
	rt-users <linux-rt-users@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	arjan <arjan@...radead.org>
Subject: Re: [PATCH RFC] pm_qos_requirement might sleep

On Thu, 2008-08-14 at 08:52 -0700, mark gross wrote:

> Keeping a lock around the different "target_value"s may not be so
> important.  Its just a 32bit scaler value, and perhaps we can make it an
> atomic type?  That way we loose the raw_spinlock.

My suggestion was to keep the locking for the write side - so as to
avoid stuff stomping on one another, but drop the read side as:

 spin_lock
 foo = var;
 spin_unlock
 return foo;

is kinda useless, it doesn't actually serialize against the usage of
foo, that is, once it gets used, var might already have acquired a new
value.

The only thing it would protect is reading var, but since that is a
machine sized read, its atomic anyway (assuming its naturally aligned).

So no need for atomic_t (its read-side is just a read too), just drop
the whole lock usage from pq_qos_requirement().

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