Date: Mon, 12 Oct 2009 12:22:46 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Bryan Donlan <bdonlan@gmail.com>, linux-kernel@vger.kernel.org,
       Ulrich Drepper <drepper@redhat.com>, linux-api@vger.kernel.org,
       Timo Sirainen <tss@iki.fi>
Subject: Re: [resend][PATCH] Added PR_SET_PROCTITLE_AREA option for prctl()
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On Tue, 13 Oct 2009 04:03:45 +0900 (JST)
KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:

> > Start simple.  What's wrong with mutex_lock() on the reader and writer
> > sides?  rwsems might be OK too.
> > 
> > In both cases we should think about whether persistent readers can
> > block the writer excessively though.
> 
> I thought your mention seems reasonable. then I mesured various locking
> performance.
> 
> 		no-contention	read-read contetion	read-write contention
> w/o patch	4627 ms		 7575 ms		 N/A
> mutex		5717 ms		33872 ms (!)		14793 ms
> rw-semaphoe	6846 ms		10734 ms		36156 ms (!)
> seqlock		4754 ms		 7558 ms		 9373 ms
> 
> Umm, seqlock is significantly better than other.

Sure, but even the worst case there is 1,000,000 operations in 34
seconds (yes?). 33 microseconds for a /proc read while under a specific
local DoS attack is OK!

If so then all implementations are acceptable and we should choose the
simplest, most-obviously-correct one.


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