linux-kernel - Re: [patch] irq threading: fix PF

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Date:	Thu, 12 Feb 2009 09:42:22 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Clark Williams <williams@...hat.com>
Cc:	Thomas Gleixner <tglx@...x.de>,
	LKML <linux-kernel@...r.kernel.org>,
	rt-users <linux-rt-users@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Carsten Emde <ce@...g.ch>
Subject: Re: [patch] irq threading: fix PF_HARDIRQ definition


* Ingo Molnar <mingo@...e.hu> wrote:

> hardirq_count() is correct, but looking at PF_HARDIRQ's definition in sched.h:
> 
>  #define PF_EXITPIDONE  0x00000008      /* pi exit done on shut down */
>  #define PF_VCPU        0x00000010      /* I'm a virtual CPU */
>  #define PF_HARDIRQ     0x08000020      /* hardirq context */
>  #define PF_NOSCHED     0x00000020      /* Userspace does not expect scheduling */
>  #define PF_FORKNOEXEC  0x00000040      /* forked but didn't exec */
> 
> Reveals that due to a typo it not only overlaps the PF_NOSCHED bit, but
> also has a spurious 0x08000000 component.

The reason is that when we forward ported the definition, i first moved it
to the 0x08000000 slot - but that slot was already taken. (our PF_ task
flag space is really crowded ...)

Then i moved it to a free spot, 0x20. Or so i thought: a later -rt patch
in the queue introduced PF_NOSCHED which overlapped it.

But the bigger problem was the spurious 0x08000000 component, which overlaps
with:

 #define PF_SOFTIRQ      0x08000000      /* softirq context */

Explaining why the warning triggered in ksoftirqd ;-)

Anyway, my fix should solve this. Do you still see the lockup under X? (make
sure you also have the IPI fix applied, see the patch in this same thread.)

	Ingo
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