linux-kernel - Re: [PATCH] proc: readdir race fix.

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Date:	Tue, 5 Sep 2006 10:30:10 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	ebiederm@...ssion.com (Eric W. Biederman)
Cc:	linux-kernel@...r.kernel.org, akpm@...l.org,
	saito.tadashi@...t.fujitsu.com, ak@...e.de, oleg@...sign.ru,
	jdelvare@...e.de
Subject: Re: [PATCH] proc: readdir race fix.

Hi,

On Mon, 04 Sep 2006 17:13:10 -0600
ebiederm@...ssion.com (Eric W. Biederman) wrote:
> These better semantics are implemented by scanning through the
> pids in numerical order and by making the file offset a pid
> plus a fixed offset.
I think this is very sane/solid approach.
Maybe this is the way to go. I'll test and ack later, thank you.

> The pid scan happens on the pid bitmap, which when you look at it is
> remarkably efficient for a brute force algorithm.  Given that a typical
> cache line is 64 bytes and thus covers space for 64*8 == 200 pids.  There
> are only 40 cache lines for the entire 32K pid space.  A typical system
> will have 100 pids or more so this is actually fewer cache lines we have
> to look at to scan a linked list, and the worst case of having to scan
> the entire pid bitmap is pretty reasonable.

I agree with you but..
Becasue this approach has to access *all* task structs in a system,
and have to scan pidhash many times. I'm not sure that this scan & lookup
is good for future implementation. But this patch is obviously better than
current implementation.



>  /*
> + * Used by proc to find the pid with the first
> + * pid that is greater than or equal to number.
> + *
> + * If there is a pid at nr this function is exactly the same as find_pid.
> + */
> +struct pid *find_next_pid(int nr)
> +{

How about find_first_used_pid(int nr) ?

-Kame

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