linux-kernel - Re: [PATCH] ioprio: grab rcu_read_lock in sys_ioprio

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Date:	Mon, 15 Nov 2010 10:15:26 +0100
From:	Jens Axboe <axboe@...nel.dk>
To:	Greg Thelen <gthelen@...gle.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] ioprio: grab rcu_read_lock in sys_ioprio_{set,get}()

On 2010-11-12 08:32, Greg Thelen wrote:
> Using:
> - CONFIG_LOCKUP_DETECTOR=y
> - CONFIG_PREEMPT=y
> - CONFIG_LOCKDEP=y
> - CONFIG_PROVE_LOCKING=y
> - CONFIG_PROVE_RCU=y
> found a missing rcu lock during boot on a 512 MiB x86_64 ubuntu vm:
>   ===================================================
>   [ 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 = 0
>   1 lock held by ureadahead/1355:
>    #0:  (tasklist_lock){.+.+..}, at: [<ffffffff8115bc09>] sys_ioprio_set+0x7f/0x29e
> 
>   stack backtrace:
>   Pid: 1355, comm: ureadahead Not tainted 2.6.37-dbg-DEV #1
>   Call Trace:
>    [<ffffffff8109c10c>] lockdep_rcu_dereference+0xaa/0xb3
>    [<ffffffff81088cbf>] find_task_by_pid_ns+0x44/0x5d
>    [<ffffffff81088cfa>] find_task_by_vpid+0x22/0x24
>    [<ffffffff8115bc3e>] sys_ioprio_set+0xb4/0x29e
>    [<ffffffff8147cf21>] ? trace_hardirqs_off_thunk+0x3a/0x3c
>    [<ffffffff8105c409>] sysenter_dispatch+0x7/0x2c
>    [<ffffffff8147cee2>] ? trace_hardirqs_on_thunk+0x3a/0x3f
> 
> The fix is to:
> a) grab rcu lock in sys_ioprio_{set,get}() and
> b) avoid grabbing tasklist_lock.
> Discussion in: http://marc.info/?l=linux-kernel&m=128951324702889

Thanks Greg, applied.

-- 
Jens Axboe

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