netdev - Re: [PATCH 05/17] mm: allow PF_MEMALLOC from softirq context

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Date:	Fri, 11 May 2012 00:39:51 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	mgorman@...e.de
Cc:	akpm@...ux-foundation.org, linux-mm@...ck.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	neilb@...e.de, a.p.zijlstra@...llo.nl, michaelc@...wisc.edu,
	emunson@...bm.net
Subject: Re: [PATCH 05/17] mm: allow PF_MEMALLOC from softirq context

From: Mel Gorman <mgorman@...e.de>
Date: Thu, 10 May 2012 14:44:58 +0100

> This is needed to allow network softirq packet processing to make
> use of PF_MEMALLOC.
> 
> Currently softirq context cannot use PF_MEMALLOC due to it not being
> associated with a task, and therefore not having task flags to fiddle
> with - thus the gfp to alloc flag mapping ignores the task flags when
> in interrupts (hard or soft) context.
> 
> Allowing softirqs to make use of PF_MEMALLOC therefore requires some
> trickery.  We basically borrow the task flags from whatever process
> happens to be preempted by the softirq.
> 
> So we modify the gfp to alloc flags mapping to not exclude task flags
> in softirq context, and modify the softirq code to save, clear and
> restore the PF_MEMALLOC flag.
> 
> The save and clear, ensures the preempted task's PF_MEMALLOC flag
> doesn't leak into the softirq. The restore ensures a softirq's
> PF_MEMALLOC flag cannot leak back into the preempted process.
> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Signed-off-by: Mel Gorman <mgorman@...e.de>

We're now making changes to task->flags from both base and
softirq context, but with non-atomic operations and no other
kind of synchronization.

As far as I can tell, this has to be racy.

If this works via some magic combination of invariants, you
absolutely have to document this, verbosely.
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