linux-kernel - Re: [PATCH 11/11] Use down_read_critical() for /proc/<pid>/exe and /proc/<pid>/maps files

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Date:	Thu, 27 May 2010 07:12:05 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Michel Lespinasse <walken@...gle.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	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 11/11] Use down_read_critical() for /proc/<pid>/exe and
 /proc/<pid>/maps files

On Mon, 24 May 2010 13:31:21 -0700
Michel Lespinasse <walken@...gle.com> wrote:

> This helps in the following situation:
> - Thread A takes a page fault while reading or writing memory.
>   do_page_fault() acquires the mmap_sem for read and blocks on disk
>   (either reading the page from file, or hitting swap) for a long
> time.
> - Thread B does an mmap call and blocks trying to acquire the mmap_sem
>   for write
> - 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.
> 
> Test results with down_read_critical_test (10 seconds):


this is a really bad idea btw

we've had many issues in the past, when this was an unfair lock, that
"top" or other similar things, caused basically a DoS......
now any process that can get to /proc/<pid>/exe or maps, can do this in
a tight enough loop so that the actual process will never get the lock
for write. BAD IDEA ;-)


-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org
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