Re: [RFC,PATCH] Use RCU to protect tasklist for unicast signals

[Ingo Molnar <mingo@elte.hu>]

* Paul E. McKenney <paulmck@us.ibm.com> wrote:

> My tests are not finding even glaring races, so time to go and create 
> some torture tests before getting too much more elaborate.  10,000 
> eyes are nice (and Oleg's eyes do seem to be working especially well), 
> but a good software-test sledgehammer has its uses as well.

i've merged this to the -rt tree, and find below a delta patch relative 
to the previous patch.

	Ingo

Index: linux/kernel/exit.c
===================================================================
--- linux.orig/kernel/exit.c
+++ linux/kernel/exit.c
@@ -74,7 +74,6 @@ repeat: 
 		__ptrace_unlink(p);
 	BUG_ON(!list_empty(&p->ptrace_list) || !list_empty(&p->ptrace_children));
 	__exit_signal(p);
-	__exit_sighand(p);
 	/*
 	 * Note that the fastpath in sys_times depends on __exit_signal having
 	 * updated the counters before a task is removed from the tasklist of
Index: linux/kernel/signal.c
===================================================================
--- linux.orig/kernel/signal.c
+++ linux/kernel/signal.c
@@ -328,17 +328,19 @@ void __exit_sighand(struct task_struct *
 	struct sighand_struct * sighand = tsk->sighand;
 
 	/* Ok, we're done with the signal handlers */
-	spin_lock(&sighand->siglock);
 	tsk->sighand = NULL;
 	if (atomic_dec_and_test(&sighand->count))
 		sighand_free(sighand);
-	spin_unlock(&sighand->siglock);
 }
 
 void exit_sighand(struct task_struct *tsk)
 {
 	write_lock_irq(&tasklist_lock);
-	__exit_sighand(tsk);
+	spin_lock(&tsk->sighand->siglock);
+	if (tsk->sighand != NULL) {
+		__exit_sighand(tsk);
+	}
+	spin_unlock(&tsk->sighand->siglock);
 	write_unlock_irq(&tasklist_lock);
 }
 
@@ -361,6 +363,7 @@ void __exit_signal(struct task_struct *t
 		if (tsk == sig->curr_target)
 			sig->curr_target = next_thread(tsk);
 		tsk->signal = NULL;
+		__exit_sighand(tsk);
 		spin_unlock(&sighand->siglock);
 		flush_sigqueue(&sig->shared_pending);
 	} else {
@@ -392,6 +395,7 @@ void __exit_signal(struct task_struct *t
 		sig->nvcsw += tsk->nvcsw;
 		sig->nivcsw += tsk->nivcsw;
 		sig->sched_time += tsk->sched_time;
+		__exit_sighand(tsk);
 		spin_unlock(&sighand->siglock);
 		sig = NULL;	/* Marker for below.  */
 	}