Re: [PATCH 05/10] perf/uprobe: SRCU-ify uprobe->consumer list

[Peter Zijlstra]

On Tue, Jul 09, 2024 at 03:33:49PM +0200, Oleg Nesterov wrote:
> On 07/09, Peter Zijlstra wrote:
> >
> > > +	guard(srcu)(&uprobes_srcu);
> > > +
> > > +	for_each_consumer_rcu(uc, uprobe->consumers) {
> > >  		int rc = 0;
> > >
> > >  		if (uc->handler) {
> > > @@ -2116,7 +2126,6 @@ static void handler_chain(struct uprobe
> > >  		WARN_ON(!uprobe_is_active(uprobe));
> > >  		unapply_uprobe(uprobe, current->mm);
> >
> >    ^^^ this remove case needs more thought.
> 
> Yeah... that is why the current code doesn't use ->consumer_rwsem, iirc.

AFAICT something like the below should work. Concurrent
remove_breakpoint() should already be possible today and doesn't appear
to be a problem.

--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -1260,6 +1260,10 @@ int uprobe_apply(struct inode *inode, lo
 	return ret;
 }
 
+/*
+ * Can race against uprobe_unregister() / register_for_each_vma(), and relies
+ * on duplicate remove_breakpoint() being a no-op.
+ */
 static int unapply_uprobe(struct uprobe *uprobe, struct mm_struct *mm)
 {
 	VMA_ITERATOR(vmi, mm, 0);
@@ -2101,6 +2105,7 @@ static void handler_chain(struct uprobe
 	struct uprobe_consumer *uc;
 	int remove = UPROBE_HANDLER_REMOVE;
 	bool need_prep = false; /* prepare return uprobe, when needed */
+	bool had_handler = false;
 
 	down_read(&uprobe->register_rwsem);
 	for (uc = uprobe->consumers; uc; uc = uc->next) {
@@ -2115,16 +2120,26 @@ static void handler_chain(struct uprobe
 		if (uc->ret_handler)
 			need_prep = true;
 
+		/*
+		 * A single handler that does not mask out REMOVE, means the
+		 * probe stays.
+		 */
+		had_handler = true;
 		remove &= rc;
 	}
 
+	/*
+	 * If there were no handlers called, nobody asked for it to be removed
+	 * but also nobody got to mask the value. Fix it up.
+	 */
+	if (!had_handler)
+		remove = 0;
+
 	if (need_prep && !remove)
 		prepare_uretprobe(uprobe, regs); /* put bp at return */
 
-	if (remove && uprobe->consumers) {
-		WARN_ON(!uprobe_is_active(uprobe));
+	if (remove)
 		unapply_uprobe(uprobe, current->mm);
-	}
 	up_read(&uprobe->register_rwsem);
 }