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


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