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

Paul E. McKenney
2010-11-08T13:52:19Z

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