On Tue, 22 Jan 2008, Stefan Richter wrote: > 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 <[EMAIL PROTECTED]> > > 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 <[EMAIL PROTECTED]> > Tested-by: Stefan Richter <[EMAIL PROTECTED]> > --- > 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); \ >
Looks fine to me. Acked-by: Davide Libenzi <[EMAIL PROTECTED]> - Davide -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
