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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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