posix-timers: sys_timer_create: simplify and s/tasklist/rcu/

- Change the code to do rcu_read_lock() instead of taking tasklist_lock,
  it is safe to get_task_struct(p) if p was found under RCU.

  However, now we must not use process's sighand/signal, they may be NULL.
  We can use current->sighand/signal instead, this "process" must belong
  to the current's thread-group.

- Factor out the common code for 2 "if (timer_event_spec)" branches, the
  !timer_event_spec case can use current too.

- use spin_lock_irq() instead of _irqsave(), kill "flags".

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: mingo@elte.hu
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

diff --git a/kernel/posix-timers.c b/kernel/posix-timers.c
index 60b2620..5b76190 100644 (file)
--- a/kernel/posix-timers.c
+++ b/kernel/posix-timers.c
@@ -463,7 +463,6 @@ sys_timer_create(const clockid_t which_clock,
        struct k_itimer *new_timer;
        int new_timer_id;
        struct task_struct *process;
-       unsigned long flags;
        sigevent_t event;
        int it_id_set = IT_ID_NOT_SET;
 
@@ -521,16 +520,11 @@ sys_timer_create(const clockid_t which_clock,
                new_timer->it_sigev_signo = event.sigev_signo;
                new_timer->it_sigev_value = event.sigev_value;
 
-               read_lock(&tasklist_lock);
-               if ((process = good_sigevent(&event))) {
+               rcu_read_lock();
+               process = good_sigevent(&event);
+               if (process)
                        get_task_struct(process);
-                       spin_lock_irqsave(&process->sighand->siglock, flags);
-                       new_timer->it_process = process;
-                       list_add(&new_timer->list,
-                               &process->signal->posix_timers);
-                       spin_unlock_irqrestore(&process->sighand->siglock, flags);
-               }
-               read_unlock(&tasklist_lock);
+               rcu_read_unlock();
                if (!process) {
                        error = -EINVAL;
                        goto out;
@@ -541,19 +535,18 @@ sys_timer_create(const clockid_t which_clock,
                new_timer->it_sigev_value.sival_int = new_timer->it_id;
                process = current->group_leader;
                get_task_struct(process);
-               spin_lock_irqsave(&process->sighand->siglock, flags);
-               new_timer->it_process = process;
-               list_add(&new_timer->list, &process->signal->posix_timers);
-               spin_unlock_irqrestore(&process->sighand->siglock, flags);
        }
 
+       spin_lock_irq(&current->sighand->siglock);
+       new_timer->it_process = process;
+       list_add(&new_timer->list, &current->signal->posix_timers);
+       spin_unlock_irq(&current->sighand->siglock);
        /*
         * In the case of the timer belonging to another task, after
         * the task is unlocked, the timer is owned by the other task
         * and may cease to exist at any time.  Don't use or modify
         * new_timer after the unlock call.
         */
-
 out:
        if (error)
                release_posix_timer(new_timer, it_id_set);