Messages in this thread Patch in this message | | | Date | Mon, 02 Sep 2019 17:36:54 +0200 | From | Roman Penyaev <> | Subject | Re: [PATCH RESEND] fs/epoll: fix the edge-triggered mode for nested epoll |
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Hi,
This is indeed a bug. (quick side note: could you please remove efd[1] from your test, because it is not related to the reproduction of a current bug).
Your patch lacks a good description, what exactly you've fixed. Let me speak out loud and please correct me if I'm wrong, my understanding of epoll internals has become a bit rusty: when epoll fds are nested an attempt to harvest events (ep_scan_ready_list() call) produces a second (repeated) event from an internal fd up to an external fd:
epoll_wait(efd[0], ...): ep_send_events(): ep_scan_ready_list(depth=0): ep_send_events_proc(): ep_item_poll(): ep_scan_ready_list(depth=1): ep_poll_safewake(): ep_poll_callback() list_add_tail(&epi, &epi->rdllist); ^^^^^^ repeated event
In your patch you forbid wakeup for the cases, where depth != 0, i.e. for all nested cases. That seems clear. But what if we can go further and remove the whole chunk, which seems excessive:
@@ -885,26 +886,11 @@ static __poll_t ep_scan_ready_list(struct eventpoll *ep,
- - if (!list_empty(&ep->rdllist)) { - /* - * Wake up (if active) both the eventpoll wait list and - * the ->poll() wait list (delayed after we release the lock). - */ - if (waitqueue_active(&ep->wq)) - wake_up(&ep->wq); - if (waitqueue_active(&ep->poll_wait)) - pwake++; - } write_unlock_irq(&ep->lock);
if (!ep_locked) mutex_unlock(&ep->mtx);
- /* We have to call this outside the lock */ - if (pwake) - ep_poll_safewake(&ep->poll_wait);
I reason like that: by the time we've reached the point of scanning events for readiness all wakeups from ep_poll_callback have been already fired and new events have been already accounted in ready list (ep_poll_callback() calls the same ep_poll_safewake()). Here, frankly, I'm not 100% sure and probably missing some corner cases.
Thoughts?
PS. You call list_empty(&ep->rdllist) without ep->lock taken, that is fine, but you should be _careful_, so list_empty_careful(&ep->rdllist) call instead. -- Roman
On 2019-09-02 07:20, hev wrote: > From: Heiher <r@hev.cc> > > The structure of event pools: > efd[1]: { efd[2] (EPOLLIN) } efd[0]: { efd[2] (EPOLLIN | > EPOLLET) } > | | > +-----------------+-----------------+ > | > v > efd[2]: { sfd[0] (EPOLLIN) } > > When sfd[0] to be readable: > * the epoll_wait(efd[0], ..., 0) should return efd[2]'s events on > first call, > and returns 0 on next calls, because efd[2] is added in > edge-triggered mode. > * the epoll_wait(efd[1], ..., 0) should returns efd[2]'s events on > every calls > until efd[2] is not readable (epoll_wait(efd[2], ...) => 0), because > efd[1] > is added in level-triggered mode. > * the epoll_wait(efd[2], ..., 0) should returns sfd[0]'s events on > every calls > until sfd[0] is not readable (read(sfd[0], ...) => EAGAIN), because > sfd[0] > is added in level-triggered mode. > > Test code: > #include <stdio.h> > #include <unistd.h> > #include <sys/epoll.h> > #include <sys/socket.h> > > int main(int argc, char *argv[]) > { > int sfd[2]; > int efd[3]; > int nfds; > struct epoll_event e; > > if (socketpair(AF_UNIX, SOCK_STREAM, 0, sfd) < 0) > goto out; > > efd[0] = epoll_create(1); > if (efd[0] < 0) > goto out; > > efd[1] = epoll_create(1); > if (efd[1] < 0) > goto out; > > efd[2] = epoll_create(1); > if (efd[2] < 0) > goto out; > > e.events = EPOLLIN; > if (epoll_ctl(efd[2], EPOLL_CTL_ADD, sfd[0], &e) < 0) > goto out; > > e.events = EPOLLIN; > if (epoll_ctl(efd[1], EPOLL_CTL_ADD, efd[2], &e) < 0) > goto out; > > e.events = EPOLLIN | EPOLLET; > if (epoll_ctl(efd[0], EPOLL_CTL_ADD, efd[2], &e) < 0) > goto out; > > if (write(sfd[1], "w", 1) != 1) > goto out; > > nfds = epoll_wait(efd[0], &e, 1, 0); > if (nfds != 1) > goto out; > > nfds = epoll_wait(efd[0], &e, 1, 0); > if (nfds != 0) > goto out; > > nfds = epoll_wait(efd[1], &e, 1, 0); > if (nfds != 1) > goto out; > > nfds = epoll_wait(efd[1], &e, 1, 0); > if (nfds != 1) > goto out; > > nfds = epoll_wait(efd[2], &e, 1, 0); > if (nfds != 1) > goto out; > > nfds = epoll_wait(efd[2], &e, 1, 0); > if (nfds != 1) > goto out; > > close(efd[2]); > close(efd[1]); > close(efd[0]); > close(sfd[0]); > close(sfd[1]); > > printf("PASS\n"); > return 0; > > out: > printf("FAIL\n"); > return -1; > } > > Cc: Al Viro <viro@ZenIV.linux.org.uk> > Cc: Andrew Morton <akpm@linux-foundation.org> > Cc: Davide Libenzi <davidel@xmailserver.org> > Cc: Davidlohr Bueso <dave@stgolabs.net> > Cc: Dominik Brodowski <linux@dominikbrodowski.net> > Cc: Eric Wong <e@80x24.org> > Cc: Jason Baron <jbaron@akamai.com> > Cc: Linus Torvalds <torvalds@linux-foundation.org> > Cc: Roman Penyaev <rpenyaev@suse.de> > Cc: Sridhar Samudrala <sridhar.samudrala@intel.com> > Cc: linux-kernel@vger.kernel.org > Cc: linux-fsdevel@vger.kernel.org > Signed-off-by: hev <r@hev.cc> > --- > fs/eventpoll.c | 6 +++++- > 1 file changed, 5 insertions(+), 1 deletion(-) > > diff --git a/fs/eventpoll.c b/fs/eventpoll.c > index d7f1f5011fac..a44cb27c636c 100644 > --- a/fs/eventpoll.c > +++ b/fs/eventpoll.c > @@ -672,6 +672,7 @@ static __poll_t ep_scan_ready_list(struct eventpoll > *ep, > { > __poll_t res; > int pwake = 0; > + int nwake = 0; > struct epitem *epi, *nepi; > LIST_HEAD(txlist); > > @@ -685,6 +686,9 @@ static __poll_t ep_scan_ready_list(struct eventpoll > *ep, > if (!ep_locked) > mutex_lock_nested(&ep->mtx, depth); > > + if (!depth || list_empty(&ep->rdllist)) > + nwake = 1; > + > /* > * Steal the ready list, and re-init the original one to the > * empty list. Also, set ep->ovflist to NULL so that events > @@ -739,7 +743,7 @@ static __poll_t ep_scan_ready_list(struct eventpoll > *ep, > list_splice(&txlist, &ep->rdllist); > __pm_relax(ep->ws); > > - if (!list_empty(&ep->rdllist)) { > + if (nwake && !list_empty(&ep->rdllist)) { > /* > * Wake up (if active) both the eventpoll wait list and > * the ->poll() wait list (delayed after we release the lock).
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