linux-kernel - Re: [patch 2/6 -rt] powerpc 2.6.20-rt8: to convert spinlocks to raw ones.

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Message-ID: <17903.33399.786962.498809@cargo.ozlabs.ibm.com>
Date:	Thu, 8 Mar 2007 14:26:47 +1100
From:	Paul Mackerras <paulus@...ba.org>
To:	Bill Huey (hui) <billh@...ppy.monkey.org>
Cc:	Sergei Shtylyov <sshtylyov@...mvista.com>,
	Tsutomu OWA <tsutomu.owa@...hiba.co.jp>,
	linuxppc-dev@...abs.org, mingo@...e.hu,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 2/6 -rt] powerpc 2.6.20-rt8: to convert spinlocks to raw ones.

Bill Huey (hui) writes:

> The places that need to be reverted to raw spinlocks are generally either
> acquired by function calls that allocate the spinlock at a terminal of the
> kernel's lock graph or isolated from other callers completely (parts of the
> timer for logic for instance). It's all about the collision of various lock
> (preemptive and non-preemptive) subtrees and how to avoid scheduling within
> atomic violations that lead to deadlocks. The -rt patch gets arbitrary
> preemption abilities by shrinking the non-preemptive sub-tree bit to the bare
> essentials of what will let a system to run yet still preserve all of
> the expected locking semantics of a critical section.

Thanks; that's an interesting explanation.

It misses the point of what I was saying to Sergei, though, which was
*not* "I don't understand your patch", it was "if this patch goes into
a git tree, someone coming along in 3 years time won't understand the
patch."  In other words I was ranting about the need for a decent
description to accompany the patch itself, so it would go into the
permanent record.

Regards,
Paul.
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