[PATCH] run_posix_cpu_timers: remove a bogus BUG_ON()

do_exit() clears ->it_##clock##_expires, but nothing prevents
another cpu to attach the timer to exiting process after that.
arm_timer() tries to protect against this race, but the check
is racy.

After exit_notify() does 'write_unlock_irq(&tasklist_lock)' and
before do_exit() calls 'schedule() local timer interrupt can find
tsk->exit_state != 0. If that state was EXIT_DEAD (or another cpu
does sys_wait4) interrupted task has ->signal == NULL.

At this moment exiting task has no pending cpu timers, they were
cleanuped in __exit_signal()->posix_cpu_timers_exit{,_group}(),
so we can just return from irq.

John Stultz recently confirmed this bug, see

	http://marc.theaimsgroup.com/?l=linux-kernel&m=115015841413687Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/exit.c

@@ -881,14 +881,6 @@ fastcall NORET_TYPE void do_exit(long code)
 
 	tsk->flags |= PF_EXITING;
 
-	/*
-	 * Make sure we don't try to process any timer firings
-	 * while we are already exiting.
-	 */
- 	tsk->it_virt_expires = cputime_zero;
- 	tsk->it_prof_expires = cputime_zero;
-	tsk->it_sched_expires = 0;
-
 	if (unlikely(in_atomic()))
 		printk(KERN_INFO "note: %s[%d] exited with preempt_count %d\n",
 				current->comm, current->pid,

kernel/posix-cpu-timers.c

@@ -1288,12 +1288,11 @@ void run_posix_cpu_timers(struct task_struct *tsk)
 
 #undef	UNEXPIRED
 
-	BUG_ON(tsk->exit_state);
-
 	/*
 	 * Double-check with locks held.
 	 */
 	read_lock(&tasklist_lock);
+	if (likely(tsk->signal != NULL)) {
 		spin_lock(&tsk->sighand->siglock);
 
 		/*
@@ -1312,6 +1311,7 @@ void run_posix_cpu_timers(struct task_struct *tsk)
 		 * spin until we've taken care of that timer below.
 		 */
 		spin_unlock(&tsk->sighand->siglock);
+	}
 	read_unlock(&tasklist_lock);
 
 	/*