[PATCH 2/6] [-mm] epoll: don't use current in irq context

[PATCH 2/6] [-mm] epoll: don't use current in irq context

[Tony Battersby]

ep_call_nested() (formerly ep_poll_safewake()) uses "current" to detect
callback recursion, but it may be called from irq context where the
use of current is generally frowned upon.  It would be better to use
get_cpu() and put_cpu() to detect the callback recursion.

Signed-off-by: Tony Battersby <tonyb@cybernetics.com>
---

This patch is against the current -mm tree, and might not apply
to 2.6.29.  Since this patch probably won't go into 2.6.29, you may
apply this version of the patch after the other patches in -mm.

--- 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;
 }
 

Re: [PATCH 2/6] [-mm] epoll: don't use current in irq context

[Davide Libenzi]

On Tue, 24 Feb 2009, Tony Battersby wrote:

> ep_call_nested() (formerly ep_poll_safewake()) uses "current" to detect
> callback recursion, but it may be called from irq context where the
> use of current is generally frowned upon.  It would be better to use
> get_cpu() and put_cpu() to detect the callback recursion.

Oh, I forgot about this one ...
We're not actually dereferencing anything in current, by using it as a 
cookie. But yes, getting the CPU is better, even at preemption expenses.
Thank you for your report Tony.


Acked-by: Davide Libenzi <davidel@xmailserver.org>


- Davide