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

[Jens Axboe <jaxboe@fusionio.com>]

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.

-- 
Jens Axboe