linux-kernel - Re: [PATCH 12/12] Use down_read_unfair() for /sys/<pid>/exe and /sys/<pid>/maps files

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Date:	Wed, 12 May 2010 14:10:38 +0100
From:	David Howells <dhowells@...hat.com>
To:	Michel Lespinasse <walken@...gle.com>
Cc:	dhowells@...hat.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Waychison <mikew@...gle.com>,
	Suleiman Souhlal <suleiman@...gle.com>,
	Ying Han <yinghan@...gle.com>
Subject: Re: [PATCH 12/12] Use down_read_unfair() for /sys/<pid>/exe and /sys/<pid>/maps files

Michel Lespinasse <walken@...gle.com> wrote:

> - Thread C is a monitoring process trying to read every /proc/pid/maps
>   in the system. This requires acquiring the mmap_sem for read. Thread C
>   blocks behind B, waiting for A to release the rwsem.  If thread C
>   could be allowed to run in parallel with A, it would probably get done
>   long before thread A's disk access completes, thus not actually slowing
>   down thread B.

Is it possible for someone to execute a DoS attack on another process using
a bunch of threads reading /proc/pid/maps?

David
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