linux-kernel - Re: [PATCH 0/4]: Respin local_irq_*

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Message-ID: <1271147834.4807.980.camel@twins>
Date:	Tue, 13 Apr 2010 10:37:14 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	David Miller <davem@...emloft.net>
Cc:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	sparclinux@...r.kernel.org
Subject: Re: [PATCH 0/4]: Respin local_irq_*_nmi() stuff.

On Tue, 2010-04-13 at 00:56 -0700, David Miller wrote:

> If we are in an NMI then doing a plain raw_local_irq_disable() will
> write PIL_NORMAL_MAX into %pil, which is lower than PIL_NMI, and thus
> we'll re-enable NMIs and recurse.
> 
> Doing a simple:
> 
> 	%pil = %pil | PIL_NORMAL_MAX
> 
> does what we want, if we're already at PIL_NMI (15) we leave it at
> that setting, else we set it to PIL_NORMAL_MAX (14).

Ah indeed, and without a conditional, very nice! 

It does rely on the exact values of the PIL_levels, it might make sense
to note that in the comment, something like:

  * Assumes: PIL_NMI | PIL_NORMAL_MAX == PIL_NMI.

Hmm, it also assumes %pil is never anything other than 0,
PIL_NORMAL_MAX, PIL_NMI, because if:

  (%pil & 1) && (%pil != PIL_NMI)

then you'll end up disabling NMIs. Could something like that ever
happen?


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