Let's cc Davide. On Mon, 11 Jun 2012 15:34:49 -0700 Paton Lewis <palewis@xxxxxxxxx> wrote: > > It is not currently possible to reliably delete epoll items when using the > same epoll set from multiple threads. After calling epoll_ctl with > EPOLL_CTL_DEL, another thread might still be executing code related to an > event for that epoll item (in response to epoll_wait). Therefore the deleting > thread does not know when it is safe to delete resources pertaining to the > associated epoll item because another thread might be using those resources. We often solve this sort of thing with refcounting - the EPOLL_CTL_DEL will drop the refcount and, if that fell to zero, remove the object. So if the object is in use by another thread, that other thread will hold a refcount and that other thread will do the object removal when dropping its refcount. I don't know if that model can be used here? > The deleting thread could wait an arbitrary amount of time after calling > epoll_ctl with EPOLL_CTL_DEL and before deleting the item, but this is > inefficient and could result in the destruction of resources before another > thread is done handling an event returned by epoll_wait. > > This patch introduces the new epoll_ctl command EPOLL_CTL_DISABLE, which > disables the associated epoll item and returns -EBUSY if the epoll item is not > currently in the epoll ready queue. This allows multiple threads to use a > mutex to determine when it is safe to delete an epoll item and its associated > resources. This allows epoll items to be deleted and closed efficiently and > without error. > > This patch has been tested against the following checklist: > http://kernelnewbies.org/UpstreamMerge/SubmitChecklist > > The following psuedocode attempts to illustrate the problem as well as the > solution provided by this patch. > > > Pseudocode for the deleting thread: It would be nice to start accumulating epoll test code in tools/testing/selftests/epoll. If you have something which can be used to kick that off, please do consider preparing it. Also, a user-visible feature wuch as this should be documented in Linux manpages. So please do cc Michael Kerrisk <mtk.manpages@xxxxxxxxx> as we work on this. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
