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SubjectRe: possible recursive locking, 2.6.24-rc7
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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@chello.nl>
---
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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