[Davide Libenzi - Sat, Jan 05, 2008 at 01:35:25PM -0800] | On Sat, 5 Jan 2008, Peter Zijlstra wrote: | [...snip...] | 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. | I don't know how the lockdep code works, so I can't say about | wake_up_nested(). Although I have a feeling is not enough in this case. | A solution may be to move the call to ep_poll_safewake() (that'd become a | simple wake_up()) inside a tasklet or whatever is today trendy for delayed | work. But his kinda scares me to be honest, since epoll has already a | bunch of places where it could be asynchronously hit (plus performance | regression will need to be verified). | | | | - Davide | |
it's quite possible that i'm wrong but just interested... why in ep_poll_safewake() the assignment struct list_head *lsthead = &psw->wake_task_list; is not protected by spinlock? - Cyrill - -- 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/