[PATCH] lockdep: annotate epoll

[Stefan Richter <stefanr@s5r6.in-berlin.de>]

On 22 Jan, Stefan Richter wrote:
> On 22 Jan, Peter Zijlstra wrote:
>> Curious though that this gets reported frequently the last few weeks,
>> afaics this problem is way old.
> 
> Here is a report against Fedora's 2.6.23-0.222.rc9.git4.fc8, filed in
> October:  https://bugzilla.redhat.com/show_bug.cgi?id=323411

Upstream bug: http://bugzilla.kernel.org/show_bug.cgi?id=9786

Date: Sun, 13 Jan 2008 19:44:26 +0100
From: Peter Zijlstra <a.p.zijlstra@chello.nl>

On Sat, 2008-01-05 at 13:35 -0800, Davide Libenzi wrote:

> I remember I talked with Arjan about this time ago. Basically, since 1) 
> you can drop an epoll fd inside another epoll fd 2) callback-based wakeups 
> are used, you can see a wake_up() from inside another wake_up(), but they 
> will never refer to the same lock instance.
> Think about:
> 
> 	dfd = socket(...);
> 	efd1 = epoll_create();
> 	efd2 = epoll_create();
> 	epoll_ctl(efd1, EPOLL_CTL_ADD, dfd, ...);
> 	epoll_ctl(efd2, EPOLL_CTL_ADD, efd1, ...);
> 
> When a packet arrives to the device underneath "dfd", the net code will 
> issue a wake_up() on its poll wake list. Epoll (efd1) has installed a 
> callback wakeup entry on that queue, and the wake_up() performed by the 
> "dfd" net code will end up in ep_poll_callback(). At this point epoll 
> (efd1) notices that it may have some event ready, so it needs to wake up 
> the waiters on its poll wait list (efd2). So it calls ep_poll_safewake() 
> that ends up in another wake_up(), after having checked about the 
> recursion constraints. That are, no more than EP_MAX_POLLWAKE_NESTS, to 
> avoid stack blasting. Never hit the same queue, to avoid loops like:
> 
> 	epoll_ctl(efd2, EPOLL_CTL_ADD, efd1, ...);
> 	epoll_ctl(efd3, EPOLL_CTL_ADD, efd2, ...);
> 	epoll_ctl(efd4, EPOLL_CTL_ADD, efd3, ...);
> 	epoll_ctl(efd1, EPOLL_CTL_ADD, efd4, ...);
> 
> The code "if (tncur->wq == wq || ..." prevents re-entering the same 
> queue/lock.

Since the epoll code is very careful to not nest same instance locks
allow the recursion.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Tested-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
---
 fs/eventpoll.c       |    2 +-
 include/linux/wait.h |   16 ++++++++++++++++
 2 files changed, 17 insertions(+), 1 deletion(-)

Index: linux/fs/eventpoll.c
===================================================================
--- linux.orig/fs/eventpoll.c
+++ linux/fs/eventpoll.c
@@ -353,7 +353,7 @@ static void ep_poll_safewake(struct poll
 	spin_unlock_irqrestore(&psw->lock, flags);
 
 	/* Do really wake up now */
-	wake_up(wq);
+	wake_up_nested(wq, 1 + wake_nests);
 
 	/* Remove the current task from the list */
 	spin_lock_irqsave(&psw->lock, flags);
Index: linux/include/linux/wait.h
===================================================================
--- linux.orig/include/linux/wait.h
+++ linux/include/linux/wait.h
@@ -161,6 +161,22 @@ wait_queue_head_t *FASTCALL(bit_waitqueu
 #define	wake_up_locked(x)		__wake_up_locked((x), TASK_UNINTERRUPTIBLE | TASK_INTERRUPTIBLE)
 #define wake_up_interruptible_sync(x)   __wake_up_sync((x),TASK_INTERRUPTIBLE, 1)
 
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+/*
+ * macro to avoid include hell
+ */
+#define wake_up_nested(x, s)						\
+do {									\
+	unsigned long flags;						\
+									\
+	spin_lock_irqsave_nested(&(x)->lock, flags, (s));		\
+	wake_up_locked(x); 						\
+	spin_unlock_irqrestore(&(x)->lock, flags);			\
+} while (0)
+#else
+#define wake_up_nested(x, s)		wake_up(x)
+#endif
+
 #define __wait_event(wq, condition) 					\
 do {									\
 	DEFINE_WAIT(__wait);						\

-- 
Stefan Richter
-=====-==--- ---= =-==-
http://arcgraph.de/sr/