posix-timers: Use sighand lock instead of tasklist_lock on timer deletion

Timer deletion doesn't need the tasklist lock.
We need to protect against:

* concurrent access to the lists p->cputime_expires and
  p->sighand->cputime_expires

* task reaping that may also delete the timer list entry

* timer firing

We already hold the timer lock which protects us against concurrent
timer firing.

The rest only need the targets sighand to be locked.
So hold it and drop the use of tasklist_lock there.
Signed-off-by: NFrederic Weisbecker <fweisbec@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Kosaki Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Andrew Morton <akpm@linux-foundation.org>

kernel/posix-cpu-timers.c

@@ -377,27 +377,32 @@ static int posix_cpu_timer_create(struct k_itimer *new_timer)
  */
 static int posix_cpu_timer_del(struct k_itimer *timer)
 {
-	struct task_struct *p = timer->it.cpu.task;
 	int ret = 0;
+	unsigned long flags;
+	struct sighand_struct *sighand;
+	struct task_struct *p = timer->it.cpu.task;
 
 	WARN_ON_ONCE(p == NULL);
 
-	read_lock(&tasklist_lock);
-	if (unlikely(p->sighand == NULL)) {
+	/*
+	 * Protect against sighand release/switch in exit/exec and process/
+	 * thread timer list entry concurrent read/writes.
+	 */
+	sighand = lock_task_sighand(p, &flags);
+	if (unlikely(sighand == NULL)) {
 		/*
 		 * We raced with the reaping of the task.
 		 * The deletion should have cleared us off the list.
 		 */
 		BUG_ON(!list_empty(&timer->it.cpu.entry));
 	} else {
-		spin_lock(&p->sighand->siglock);
 		if (timer->it.cpu.firing)
 			ret = TIMER_RETRY;
 		else
 			list_del(&timer->it.cpu.entry);
-		spin_unlock(&p->sighand->siglock);
+
+		unlock_task_sighand(p, &flags);
 	}
-	read_unlock(&tasklist_lock);
 
 	if (!ret)
 		put_task_struct(p);