[PATCH 1/5] epoll: don't use current in irq context

[Tony Battersby <tonyb@cybernetics.com>]

ep_call_nested() (formerly ep_poll_safewake()) uses "current" (without
dereferencing it) to detect callback recursion, but it may be called
from irq context where the use of current is generally discouraged.  It
would be better to use get_cpu() and put_cpu() to detect the callback
recursion.

Signed-off-by: Tony Battersby <tonyb@cybernetics.com>
Acked-by: Davide Libenzi <davidel@xmailserver.org>
---

Dropped patch 1 from the original set, so patches 2 - 6 are now 1 - 5.
These should all apply to the current -mm tree.

--- a/fs/eventpoll.c	2009-02-23 13:13:33.000000000 -0500
+++ b/fs/eventpoll.c	2009-02-23 13:17:46.000000000 -0500
@@ -97,8 +97,8 @@ struct epoll_filefd {
  */
 struct nested_call_node {
 	struct list_head llink;
-	struct task_struct *task;
 	void *cookie;
+	int cpu;
 };
 
 /*
@@ -327,7 +327,7 @@ static int ep_call_nested(struct nested_
 {
 	int error, call_nests = 0;
 	unsigned long flags;
-	struct task_struct *this_task = current;
+	int this_cpu = get_cpu();
 	struct list_head *lsthead = &ncalls->tasks_call_list;
 	struct nested_call_node *tncur;
 	struct nested_call_node tnode;
@@ -340,20 +340,19 @@ static int ep_call_nested(struct nested_
 	 * very much limited.
 	 */
 	list_for_each_entry(tncur, lsthead, llink) {
-		if (tncur->task == this_task &&
+		if (tncur->cpu == this_cpu &&
 		    (tncur->cookie == cookie || ++call_nests > max_nests)) {
 			/*
 			 * Ops ... loop detected or maximum nest level reached.
 			 * We abort this wake by breaking the cycle itself.
 			 */
-			spin_unlock_irqrestore(&ncalls->lock, flags);
-
-			return -1;
+			error = -1;
+			goto out_unlock;
 		}
 	}
 
 	/* Add the current task and cookie to the list */
-	tnode.task = this_task;
+	tnode.cpu = this_cpu;
 	tnode.cookie = cookie;
 	list_add(&tnode.llink, lsthead);
 
@@ -365,8 +364,10 @@ static int ep_call_nested(struct nested_
 	/* Remove the current task from the list */
 	spin_lock_irqsave(&ncalls->lock, flags);
 	list_del(&tnode.llink);
+ out_unlock:
 	spin_unlock_irqrestore(&ncalls->lock, flags);
 
+	put_cpu();
 	return error;
 }