Messages in this thread Patch in this message | | | Subject | Re: possible recursive locking, 2.6.24-rc7 | From | Peter Zijlstra <> | Date | Sun, 13 Jan 2008 19:44:26 +0100 |
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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?
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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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