Re: [2.6.37-rc1] sys_ioprio_set and RCU locking...

[Jens Axboe]

On 2010-11-08 14:52, Paul E. McKenney wrote:
> 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?

Certainly, I'm in the middle of patch monkeying now anyway. Will queue
it up.


-- 
Jens Axboe

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