From 0ccf831cbee94df9c5006dd46248c0f07847dd7c Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Mon, 4 Feb 2008 22:27:20 -0800 Subject: [PATCH] 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 Tested-by: Stefan Richter Acked-by: Davide Libenzi Cc: Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- fs/eventpoll.c | 2 +- include/linux/wait.h | 16 ++++++++++++++++ 2 files changed, 17 insertions(+), 1 deletion(-) diff --git a/fs/eventpoll.c b/fs/eventpoll.c index 81c04abfb1aa..a415f42d32cf 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -353,7 +353,7 @@ static void ep_poll_safewake(struct poll_safewake *psw, wait_queue_head_t *wq) 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); diff --git a/include/linux/wait.h b/include/linux/wait.h index 1f4fb0a81ecd..33a2aa9e02f2 100644 --- a/include/linux/wait.h +++ b/include/linux/wait.h @@ -162,6 +162,22 @@ wait_queue_head_t *FASTCALL(bit_waitqueue(void *, int)); #define wake_up_interruptible_all(x) __wake_up(x, TASK_INTERRUPTIBLE, 0, NULL) #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); \ -- 2.20.1