On 9/19/19 5:24 AM, hev wrote:
> From: Heiher <r@xxxxxx>
>
> Take the case where we have:
>
> t0
> | (ew)
> e0
> | (et)
> e1
> | (lt)
> s0
>
> t0: thread 0
> e0: epoll fd 0
> e1: epoll fd 1
> s0: socket fd 0
> ew: epoll_wait
> et: edge-trigger
> lt: level-trigger
>
> When s0 fires an event, e1 catches the event, and then e0 catches an event from
> e1. After this, There is a thread t0 do epoll_wait() many times on e0, it should
> only get one event in total, because e1 is a dded to e0 in edge-triggered mode.
>
> This patch only allows the wakeup(&ep->poll_wait) in ep_scan_ready_list under
> two conditions:
>
> 1. depth == 0.
> 2. There have event is added to ep->ovflist during processing.
>
> Test code:
> #include <unistd.h>
> #include <sys/epoll.h>
> #include <sys/socket.h>
>
> int main(int argc, char *argv[])
> {
> int sfd[2];
> int efd[2];
> 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;
>
> e.events = EPOLLIN;
> if (epoll_ctl(efd[1], EPOLL_CTL_ADD, sfd[0], &e) < 0)
> goto out;
>
> e.events = EPOLLIN | EPOLLET;
> if (epoll_ctl(efd[0], EPOLL_CTL_ADD, efd[1], &e) < 0)
> goto out;
>
> if (write(sfd[1], "w", 1) != 1)
> goto out;
>
> if (epoll_wait(efd[0], &e, 1, 0) != 1)
> goto out;
>
> if (epoll_wait(efd[0], &e, 1, 0) != 0)
> goto out;
>
> close(efd[0]);
> close(efd[1]);
> close(sfd[0]);
> close(sfd[1]);
>
> return 0;
>
> out:
> return -1;
> }
>
> More tests:
> https://github.com/heiher/epoll-wakeup
>
> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Davide Libenzi <davidel@xxxxxxxxxxxxxxx>
> Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
> Cc: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx>
> Cc: Eric Wong <e@xxxxxxxxx>
> Cc: Jason Baron <jbaron@xxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Roman Penyaev <rpenyaev@xxxxxxx>
> Cc: Sridhar Samudrala <sridhar.samudrala@xxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Signed-off-by: hev <r@xxxxxx>
> ---
> fs/eventpoll.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> index c4159bcc05d9..fa71468dbd51 100644
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -685,6 +685,9 @@ static __poll_t ep_scan_ready_list(struct eventpoll *ep,
> if (!ep_locked)
> mutex_lock_nested(&ep->mtx, depth);
>
> + if (!depth || list_empty_careful(&ep->rdllist))
> + pwake++;
> +
> /*
> * Steal the ready list, and re-init the original one to the
> * empty list. Also, set ep->ovflist to NULL so that events
> @@ -755,7 +758,7 @@ static __poll_t ep_scan_ready_list(struct eventpoll *ep,
> mutex_unlock(&ep->mtx);
>
> /* We have to call this outside the lock */
> - if (pwake)
> + if (pwake == 2)
> ep_poll_safewake(&ep->poll_wait);
>
> return res;
>
Hi,
I was thinking more like the following. I tried it using your test-suite
and it seems to work. What do you think?
Thanks,
-Jason
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index d7f1f50..662136b 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -712,6 +712,15 @@ static __poll_t ep_scan_ready_list(struct eventpoll
*ep,
for (nepi = READ_ONCE(ep->ovflist); (epi = nepi) != NULL;
nepi = epi->next, epi->next = EP_UNACTIVE_PTR) {
/*
+ * We only need to wakeup nested epoll fds if
+ * if something has been queued to the overflow list,
+ * since the ep_poll() traverses the rdllist during
+ * recursive poll and thus events on the overflow list
+ * may not be visible yet.
+ */
+ if (!pwake)
+ pwake++;
+ /*
* We need to check if the item is already in the list.
* During the "sproc" callback execution time, items are
* queued into ->ovflist but the "txlist" might already
@@ -755,7 +764,7 @@ static __poll_t ep_scan_ready_list(struct eventpoll *ep,
mutex_unlock(&ep->mtx);
/* We have to call this outside the lock */
- if (pwake)
+ if (pwake == 2)
ep_poll_safewake(&ep->poll_wait);
return res;
