Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1754496Ab0KHNwT (ORCPT <rfc822;w@1wt.eu>);
	Mon, 8 Nov 2010 08:52:19 -0500
Received: from e4.ny.us.ibm.com ([32.97.182.144]:34811 "EHLO e4.ny.us.ibm.com"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1752037Ab0KHNwS (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Mon, 8 Nov 2010 08:52:18 -0500
Date: Mon, 8 Nov 2010 05:52:04 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Jens Axboe <jaxboe@fusionio.com>
Cc: Daniel J Blueman <daniel.blueman@gmail.com>,
        Linux Kernel <linux-kernel@vger.kernel.org>
Subject: Re: [2.6.37-rc1] sys_ioprio_set and RCU locking...
Message-ID: <20101108135204.GE2580@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
References: <AANLkTi=xj6xW5SnrdQos4P-MtJP8+fwfaa3r7LPfdLh4@mail.gmail.com>
 <20101107185433.GD15561@linux.vnet.ibm.com>
 <4CD7FAFD.1060802@fusionio.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <4CD7FAFD.1060802@fusionio.com>
User-Agent: Mutt/1.5.20 (2009-06-14)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Length: 2267
Lines: 78

On Mon, Nov 08, 2010 at 02:28:29PM +0100, Jens Axboe wrote:
> On 2010-11-07 19:54, Paul E. McKenney wrote:
> > On Tue, Nov 02, 2010 at 12:15:30PM +0000, Daniel J Blueman wrote:
> >> With 2.6.37-rc1, I observe sys_ioprio_set not taking the RCU lock [1]
> >> across access to the task credentials.
> >>
> >> Inspecting the code in fs/ioprio.c, the tasklist_lock is held for read
> >> across the __task_cred call, which is presumably sufficient to prevent
> >> the task credentials becoming stale.
> >>
> >> Thus, is there preference to take the RCU lock for read across the
> >> credential access eg at [2], or annotate the call?
> >>
> >> Thanks,
> >>   Daniel
> >>
> >> --- [1]
> >>
> >> ===================================================
> >>
> >> [ INFO: suspicious rcu_dereference_check() usage. ]
> >>
> >> ---------------------------------------------------
> >>
> >> kernel/pid.c:419 invoked rcu_dereference_check() without protection!
> >>
> >>
> >>
> >> other info that might help us debug this:
> >>
> >>
> >>
> >>
> >> rcu_scheduler_active = 1, debug_locks = 1
> >>
> >> 1 lock held by start-stop-daem/2246:
> >>
> >>  #0:  (tasklist_lock){.?.?..}, at: [<ffffffff811a2dfa>]
> >> sys_ioprio_set+0x8a/0x400
> >>
> >>
> >>
> >> stack backtrace:
> >>
> >> Pid: 2246, comm: start-stop-daem Not tainted 2.6.37-rc1-330cd+ #2
> >>
> >> Call Trace:
> >>
> >>  [<ffffffff8109f5f4>] lockdep_rcu_dereference+0xa4/0xc0
> >>
> >>  [<ffffffff81085651>] find_task_by_pid_ns+0x81/0x90
> >>
> >>  [<ffffffff8108567d>] find_task_by_vpid+0x1d/0x20
> >>
> >>  [<ffffffff811a3160>] sys_ioprio_set+0x3f0/0x400
> >>
> >>  [<ffffffff816efa79>] ? trace_hardirqs_on_thunk+0x3a/0x3f
> >>
> >>  [<ffffffff81003482>] system_call_fastpath+0x16/0x1b
> >>
> >>
> >> --- [2]
> >>
> >> Take the RCU lock for read across acquiring the pointer to the task
> >> credentials and dereferencing it.
> > 
> > Jens, does this look sane?
> 
> Yes, looks clean enough to me.

Very good!  Are you willing to take the patch in your tree?

							Thanx, Paul
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/