[PATCH 4/6] epoll: don't use current in irq context

[Paolo Pisati]

From: Tony Battersby <tonyb at 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 at cybernetics.com>
Acked-by: Davide Libenzi <davidel at xmailserver.org>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
(cherry picked from commit abff55cee1039b5a3b96f7a5eb6e65b9f247a274)

Signed-off-by: Paolo Pisati <paolo.pisati at canonical.com>
---
 fs/eventpoll.c |   15 ++++++++-------
 1 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index fe950e6..863620e 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -113,8 +113,8 @@ struct epoll_filefd {
  */
 struct nested_call_node {
 	struct list_head llink;
-	struct task_struct *task;
 	void *cookie;
+	int cpu;
 };
 
 /*
@@ -334,7 +334,7 @@ static int ep_call_nested(struct nested_calls *ncalls, int max_nests,
 {
 	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;
@@ -347,20 +347,19 @@ static int ep_call_nested(struct nested_calls *ncalls, int max_nests,
 	 * 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);
 
@@ -372,8 +371,10 @@ static int ep_call_nested(struct nested_calls *ncalls, int max_nests,
 	/* 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;
 }
 
-- 
1.7.5.4