Re: rcu_read_lock/unlock protect find_task_by_vpid call in posix_cpu_timer_create

[Peter Zijlstra <peterz@infradead.org>]

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?