[PATCH] Fix cpu timers exit deadlock and races

Oleg Nesterov reported an SMP deadlock. If there is a running timer
tracking a different process's CPU time clock when the process owning
the timer exits, we deadlock on tasklist_lock in posix_cpu_timer_del via
exit_itimers.

That code was using tasklist_lock to check for a race with __exit_signal
being called on the timer-target task and clearing its ->signal.
However, there is actually no such race. __exit_signal will have called
posix_cpu_timers_exit and posix_cpu_timers_exit_group before it does
that. Those will clear those k_itimer's association with the dying
task, so posix_cpu_timer_del will return early and never reach the code
in question.

In addition, posix_cpu_timer_del called from exit_itimers during execve
or directly from timer_delete in the process owning the timer can race
with an exiting timer-target task to cause a double put on timer-target
task struct. Make sure we always access cpu_timers lists with sighand
lock held.

Signed-off-by: Roland McGrath <[e-mail]>
Signed-off-by: Chris Wright <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/posix-cpu-timers.c

···
 	if (unlikely(p == NULL))
 		return 0;
 
+	spin_lock(&p->sighand->siglock);
 	if (!list_empty(&timer->it.cpu.entry)) {
-		read_lock(&tasklist_lock);
-		if (unlikely(p->signal == 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 {
-			/*
-			 * Take us off the task's timer list.
-			 */
-			spin_lock(&p->sighand->siglock);
-			list_del(&timer->it.cpu.entry);
-			spin_unlock(&p->sighand->siglock);
-		}
-		read_unlock(&tasklist_lock);
+		/*
+		 * Take us off the task's timer list.  We don't need to
+		 * take tasklist_lock and check for the task being reaped.
+		 * If it was reaped, it already called posix_cpu_timers_exit
+		 * and posix_cpu_timers_exit_group to clear all the timers
+		 * that pointed to it.
+		 */
+		list_del(&timer->it.cpu.entry);
+		put_task_struct(p);
 	}
-	put_task_struct(p);
+	spin_unlock(&p->sighand->siglock);
 
 	return 0;
 }