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    SubjectRe: rcu_read_lock/unlock protect find_task_by_vpid call in check_clock
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    On Wed, 2010-11-24 at 20:35 -0500, Dave Jones wrote:
    > ===================================================
    > [ 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 scrashme/13382:
    > #0: (tasklist_lock){.?.?..}, at: [<ffffffff8106ddea>] check_clock+0x46/0x9a
    >
    > stack backtrace:
    > Pid: 13382, comm: scrashme Not tainted 2.6.37-rc3+ #8
    > Call Trace:
    > [<ffffffff8107cfe1>] lockdep_rcu_dereference+0x9d/0xa5
    > [<ffffffff81069d08>] find_task_by_pid_ns+0x44/0x5d
    > [<ffffffff81069d43>] find_task_by_vpid+0x22/0x24
    > [<ffffffff8106ddf2>] check_clock+0x4e/0x9a
    > [<ffffffff8106deac>] posix_cpu_clock_getres+0x16/0x41
    > [<ffffffff8106be74>] sys_clock_getres+0x39/0xa0
    > [<ffffffff81009cb2>] system_call_fastpath+0x16/0x1b
    >
    > Signed-off-by: Dave Jones <davej@redhat.com>
    >
    > diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
    > index 6842eeb..4bef9aa 100644
    > --- a/kernel/posix-cpu-timers.c
    > +++ b/kernel/posix-cpu-timers.c
    > @@ -38,11 +38,13 @@ static int check_clock(const clockid_t which_clock)
    > return 0;
    >
    > read_lock(&tasklist_lock);
    > + rcu_read_lock();
    > p = find_task_by_vpid(pid);
    > if (!p || !(CPUCLOCK_PERTHREAD(which_clock) ?
    > same_thread_group(p, current) : thread_group_leader(p))) {
    > error = -EINVAL;
    > }
    > + rcu_read_unlock();
    > read_unlock(&tasklist_lock);
    >
    > return error;


    Pretty much the same comment as the other patch..

    <copy/paste>

    Do we still need the tasklist_lock in this case?

    Also, why is that think complaining, surely the tasklist_lock pins any
    and all PID objects?


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