uprobes: Introduce free_ret_instance()

We can simplify uprobe_free_utask() and handle_uretprobe_chain()
if we add a simple helper which does put_uprobe/kfree and
returns the ->next return_instance.

Tested-by: Pratyush Anand <panand@redhat.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Acked-by: Anton Arapov <arapov@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20150721134006.GA4740@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/events/uprobes.c

@@ -1378,14 +1378,22 @@ unsigned long uprobe_get_trap_addr(struct pt_regs *regs)
 	return instruction_pointer(regs);
 }
 
+static struct return_instance *free_ret_instance(struct return_instance *ri)
+{
+	struct return_instance *next = ri->next;
+	put_uprobe(ri->uprobe);
+	kfree(ri);
+	return next;
+}
+
 /*
  * Called with no locks held.
  * Called in context of a exiting or a exec-ing thread.
  */
 void uprobe_free_utask(struct task_struct *t)
 {
 	struct uprobe_task *utask = t->utask;
-	struct return_instance *ri, *tmp;
+	struct return_instance *ri;
 
 	if (!utask)
 		return;
@@ -1394,13 +1402,8 @@ void uprobe_free_utask(struct task_struct *t)
 		put_uprobe(utask->active_uprobe);
 
 	ri = utask->return_instances;
-	while (ri) {
-		tmp = ri;
-		ri = ri->next;
-
-		put_uprobe(tmp->uprobe);
-		kfree(tmp);
-	}
+	while (ri)
+		ri = free_ret_instance(ri);
 
 	xol_free_insn_slot(t);
 	kfree(utask);
@@ -1770,7 +1773,7 @@ handle_uretprobe_chain(struct return_instance *ri, struct pt_regs *regs)
 static bool handle_trampoline(struct pt_regs *regs)
 {
 	struct uprobe_task *utask;
-	struct return_instance *ri, *tmp;
+	struct return_instance *ri;
 	bool chained;
 
 	utask = current->utask;
@@ -1792,11 +1795,7 @@ static bool handle_trampoline(struct pt_regs *regs)
 		handle_uretprobe_chain(ri, regs);
 
 		chained = ri->chained;
-		put_uprobe(ri->uprobe);
-
-		tmp = ri;
-		ri = ri->next;
-		kfree(tmp);
+		ri = free_ret_instance(ri);
 		utask->depth--;
 
 		if (!chained)