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    SubjectRe: rcu_read_lock/unlock protect find_task_by_vpid call in posix_cpu_timer_create
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    On Wed, 2010-11-24 at 20:09 -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/20820:
    > #0: (tasklist_lock){.?.?..}, at: [<ffffffff8106e30f>] posix_cpu_timer_create+0x50/0xee
    >
    > stack backtrace:
    > Pid: 20820, comm: scrashme Not tainted 2.6.37-rc3+ #7
    > Call Trace:
    > [<ffffffff8107cfd5>] lockdep_rcu_dereference+0x9d/0xa5
    > [<ffffffff81069d08>] find_task_by_pid_ns+0x44/0x5d
    > [<ffffffff81069d43>] find_task_by_vpid+0x22/0x24
    > [<ffffffff8106e32d>] posix_cpu_timer_create+0x6e/0xee
    > [<ffffffff8106eb88>] do_cpu_nanosleep+0x83/0x1ad
    > [<ffffffff8106f50a>] posix_cpu_nsleep+0x6d/0xf6
    > [<ffffffff810f9a64>] ? might_fault+0xa5/0xac
    > [<ffffffff810f9a1b>] ? might_fault+0x5c/0xac
    > [<ffffffff8106bf57>] sys_clock_nanosleep+0x7c/0xcb
    > [<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..2658955 100644
    > --- a/kernel/posix-cpu-timers.c
    > +++ b/kernel/posix-cpu-timers.c
    > @@ -391,6 +391,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer)
    > INIT_LIST_HEAD(&new_timer->it.cpu.entry);
    >
    > read_lock(&tasklist_lock);
    > + rcu_read_lock();
    > if (CPUCLOCK_PERTHREAD(new_timer->it_clock)) {
    > if (pid == 0) {
    > p = current;
    > @@ -414,6 +415,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer)
    > } else {
    > ret = -EINVAL;
    > }
    > + rcu_read_unlock();
    > read_unlock(&tasklist_lock);
    >
    > return ret;

    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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