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SubjectRe: [PATCH] posix-cpu-timers: rcu_read_lock/unlock protect find_task_by_vpid call
On Tue, 2 Nov 2010, Sergey Senozhatsky wrote:

> Commit 4221a9918e38b7494cee341dda7b7b4bb8c04bde "Add RCU check for
> find_task_by_vpid()" introduced rcu_lockdep_assert to find_task_by_pid_ns.
> Add rcu_read_lock/rcu_read_unlock to call find_task_by_vpid.
>
> Tetsuo Handa wrote:
>
> Quoting from one of posts in that thead
> http://kerneltrap.org/mailarchive/linux-kernel/2010/2/8/4536388
>
> | Usually tasklist gives enough protection, but if copy_process() fails
> | it calls free_pid() lockless and does call_rcu(delayed_put_pid().
> | This means, without rcu lock find_pid_ns() can't scan the hash table
> | safely.

We can remove the tasklist_lock while at it. rcu_read_lock is enough.

Thanks,

tglx

>
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
>
> ---
>
> diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
> index 6842eeb..855bc53 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;
> @@ -395,17 +397,21 @@ int posix_cpu_timer_create(struct k_itimer *new_timer)
> if (pid == 0) {
> p = current;
> } else {
> + rcu_read_lock();
> p = find_task_by_vpid(pid);
> if (p && !same_thread_group(p, current))
> p = NULL;
> + rcu_read_unlock();
> }
> } else {
> if (pid == 0) {
> p = current->group_leader;
> } else {
> + rcu_read_lock();
> p = find_task_by_vpid(pid);
> if (p && !thread_group_leader(p))
> p = NULL;
> + rcu_read_unlock();
> }
> }
> new_timer->it.cpu.task = p;
>


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