[PATCH 2/3] run_posix_cpu_timers: remove a bogus BUG_ON()

[Oleg Nesterov <oleg@tv-sign.ru>]

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=115015841413687

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

--- 2.6.17-rc6/kernel/exit.c~2_BUG	2006-06-15 17:59:26.000000000 +0400
+++ 2.6.17-rc6/kernel/exit.c	2006-06-15 18:46:00.000000000 +0400
@@ -881,14 +881,6 @@ fastcall NORET_TYPE void do_exit(long co
 
 	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,
--- 2.6.17-rc6/kernel/posix-cpu-timers.c~2_BUG	2006-06-15 18:01:57.000000000 +0400
+++ 2.6.17-rc6/kernel/posix-cpu-timers.c	2006-06-15 18:46:00.000000000 +0400
@@ -1288,30 +1288,30 @@ void run_posix_cpu_timers(struct task_st
 
 #undef	UNEXPIRED
 
-	BUG_ON(tsk->exit_state);
-
 	/*
 	 * Double-check with locks held.
 	 */
 	read_lock(&tasklist_lock);
-	spin_lock(&tsk->sighand->siglock);
+	if (likely(tsk->signal != NULL)) {
+		spin_lock(&tsk->sighand->siglock);
 
-	/*
-	 * Here we take off tsk->cpu_timers[N] and tsk->signal->cpu_timers[N]
-	 * all the timers that are firing, and put them on the firing list.
-	 */
-	check_thread_timers(tsk, &firing);
-	check_process_timers(tsk, &firing);
+		/*
+		 * Here we take off tsk->cpu_timers[N] and tsk->signal->cpu_timers[N]
+		 * all the timers that are firing, and put them on the firing list.
+		 */
+		check_thread_timers(tsk, &firing);
+		check_process_timers(tsk, &firing);
 
-	/*
-	 * We must release these locks before taking any timer's lock.
-	 * There is a potential race with timer deletion here, as the
-	 * siglock now protects our private firing list.  We have set
-	 * the firing flag in each timer, so that a deletion attempt
-	 * that gets the timer lock before we do will give it up and
-	 * spin until we've taken care of that timer below.
-	 */
-	spin_unlock(&tsk->sighand->siglock);
+		/*
+		 * We must release these locks before taking any timer's lock.
+		 * There is a potential race with timer deletion here, as the
+		 * siglock now protects our private firing list.  We have set
+		 * the firing flag in each timer, so that a deletion attempt
+		 * that gets the timer lock before we do will give it up and
+		 * spin until we've taken care of that timer below.
+		 */
+		spin_unlock(&tsk->sighand->siglock);
+	}
 	read_unlock(&tasklist_lock);
 
 	/*