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>

1 files changed, 8 insertions, 15 deletions

diff --git a/kernel/posix-timers.c b/kernel/posix-timers.c
index 60b262051d1d..5b761903b49a 100644
--- 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);