Re: possible recursive locking, 2.6.24-rc7

[Peter Zijlstra]

On Sun, 2008-01-13 at 17:22 +0100, Peter Zijlstra wrote:
> On Sun, 2008-01-13 at 17:51 +0200, Denys Fedoryshchenko wrote:
> > Hi, got in dmesg
> > Not sure where to send (there is TCP), so sending netdev@ and kernel@
> 
> It's epoll, this is a known issue and will be fixed soon. Thanks for
> reporting.

If its easy for you to reproduce, would you mind giving the following
patch a spin?

---

Subject: lockdep: annotate epoll

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@xxxxxxxxx>
---
 fs/eventpoll.c       |    2 +-
 include/linux/wait.h |   16 ++++++++++++++++
 2 files changed, 17 insertions(+), 1 deletion(-)

Index: linux-2.6/fs/eventpoll.c
===================================================================
--- linux-2.6.orig/fs/eventpoll.c
+++ linux-2.6/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-2.6/include/linux/wait.h
===================================================================
--- linux-2.6.orig/include/linux/wait.h
+++ linux-2.6/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);						\


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