Re: 2.6.35 - INFO: kernel/exit.c:1387 invoked rcu_dereference_check()without protection!

[Miles Lane]

Hi Paul and friends,

Now in 2.6.36-rc1:

[    7.295797] ===================================================
[    7.295801] [ INFO: suspicious rcu_dereference_check() usage. ]
[    7.295805] ---------------------------------------------------
[    7.295810] kernel/exit.c:1390 invoked rcu_dereference_check()
without protection!
[    7.295813]
[    7.295814] other info that might help us debug this:
[    7.295816]
[    7.295819]
[    7.295820] rcu_scheduler_active = 1, debug_locks = 1
[    7.295825] 2 locks held by init/1:
[    7.295827]  #0:  (tasklist_lock){.+.+..}, at: [<ffffffff8103e31c>]
do_wait+0xa5/0x1fa
[    7.295843]  #1:  (&(&sighand->siglock)->rlock){......}, at:
[<ffffffff8103de60>] wait_consider_task+0x5e1/0x9f8
[    7.295854]
[    7.295855] stack backtrace:
[    7.295860] Pid: 1, comm: init Not tainted 2.6.36-rc1 #3
[    7.295864] Call Trace:
[    7.295872]  [<ffffffff81061999>] lockdep_rcu_dereference+0x9d/0xa6
[    7.295878]  [<ffffffff8103deef>] wait_consider_task+0x670/0x9f8
[    7.295884]  [<ffffffff8103e388>] do_wait+0x111/0x1fa
[    7.295890]  [<ffffffff8103e5b9>] sys_waitid+0x7f/0x178
[    7.295898]  [<ffffffff81002a5c>] ? sysret_check+0x27/0x62
[    7.295904]  [<ffffffff8103cbc6>] ? child_wait_callback+0x0/0x53
[    7.295911]  [<ffffffff81002a2b>] system_call_fastpath+0x16/0x1b


On Mon, Aug 9, 2010 at 10:27 AM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> On Tue, Aug 03, 2010 at 11:20:58PM -0400, Miles Lane wrote:
>> [ INFO: suspicious rcu_dereference_check() usage. ]
>> ---------------------------------------------------
>> kernel/exit.c:1387 invoked rcu_dereference_check() without protection!
>>
>> other info that might help us debug this:
>>
>> rcu_scheduler_active = 1, debug_locks = 1
>> 2 locks held by init/1:
>>  #0:  (tasklist_lock){.+.+..}, at: [<ffffffff81045ca8>] do_wait+0xa9/0x1fa
>>  #1:  (&(&sighand->siglock)->rlock){......}, at: [<ffffffff810457e8>]
>> wait_consider_task+0x5e1/0x9f8
>>
>> stack backtrace:
>> Pid: 1, comm: init Not tainted 2.6.35 #15
>> Call Trace:
>>  [<ffffffff8106759c>] lockdep_rcu_dereference+0x9d/0xa6
>>  [<ffffffff81045877>] wait_consider_task+0x670/0x9f8
>>  [<ffffffff81045d14>] do_wait+0x115/0x1fa
>>  [<ffffffff81045f41>] sys_waitid+0x7f/0x178
>>  [<ffffffff81009cba>] ? sysret_check+0x2e/0x69
>>  [<ffffffff8104454e>] ? child_wait_callback+0x0/0x53
>>  [<ffffffff81009c82>] system_call_fastpath+0x16/0x1b
>
> This one is interesting.  The ->sighand->siglock is held, but the
> rcu_dereference_check() check condition requires that either the
> task is dead or that we are in an RCU read-side critical section.
> The comment preceding the call to __task_cred() claims that we
> "don't need the RCU readlock here as we're holding a spinlock."
> This comment dates back to 2008, so might be obsolete.
>
> David, should we enclose the __task_cred() in wait_task_stopped()
> with rcu_read_lock()?  Or would it be better to add a check to
> __task_cred() checking for ->sighand->siglock?  Or do we need to
> do something else entirely?  ;-)
>
>                                                        Thanx, Paul
>
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