linux-kernel - Re: Request for documentation of PID handling functions

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Message-ID: <488FCC14.3070208@openvz.org>
Date:	Wed, 30 Jul 2008 06:04:04 +0400
From:	Pavel Emelyanov <xemul@...nvz.org>
To:	David Howells <dhowells@...hat.com>
CC:	Oleg Nesterov <oleg@...sign.ru>, jmorris@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: Request for documentation of PID handling functions

David Howells wrote:
> Hi Pavel,
> 
> Can you please provide documentation for the PID handling functions you
> supplied in commit:
> 
> 	7af5729474b5b8ad385adadab78d6e723e7655a3
> 
> In particular I'd like to know what locks are required when.  I'm trying to
> determine if in this piece of code:
> 
> 		read_lock(&tasklist_lock);
> 		ret = 0;
> 		if (pid != task_pid_vnr(current))
> 			ret = -EPERM;
> 		read_unlock(&tasklist_lock);
> 
> the lock of tasklist_lock is necessary.

The RCU protection is enough to get pids from tasks. However, the pids
are detached under the tasklist lock write-locked, so provided you're
*already* sitting under it the RCU is excessive.

However, Oleg pointed me out some time ago, that in some cases RCU-only
protection was not enough in some cases, so I put him in Cc.

> David
> 

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