Re: [PATCH RESEND] fs/epoll: fix the edge-triggered mode for nested epoll

[Heiher <r@hev.cc>]

Hi,

On Wed, Sep 4, 2019 at 8:02 PM Jason Baron <jbaron@akamai.com> wrote:
>
>
>
> On 9/4/19 5:57 AM, Roman Penyaev wrote:
> > On 2019-09-03 23:08, Jason Baron wrote:
> >> On 9/2/19 11:36 AM, Roman Penyaev wrote:
> >>> 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?
> >>
> >> So the: 'wake_up(&ep->wq);' part, I think is about waking up other
> >> threads that may be in waiting in epoll_wait(). For example, there may
> >> be multiple threads doing epoll_wait() on the same epoll fd, and the
> >> logic above seems to say thread 1 may have processed say N events and
> >> now its going to to go off to work those, so let's wake up thread 2 now
> >> to handle the next chunk.
> >
> > Not quite. Thread which calls ep_scan_ready_list() processes all the
> > events, and while processing those, removes them one by one from the
> > ready list.  But if event mask is !0 and event belongs to
> > Level Triggered Mode descriptor (let's say default mode) it tails event
> > again back to the list (because we are in level mode, so event should
> > be there).  So at the end of this traversing loop ready list is likely
> > not empty, and if so, wake up again is called for nested epoll fds.
> > But, those nested epoll fds should get already all the notifications
> > from the main event callback ep_poll_callback(), regardless any thread
> > which traverses events.
> >
> > I suppose this logic exists for decades, when Davide (the author) was
> > reshuffling the code here and there.
> >
> > But I do not feel confidence to state that this extra wakeup is bogus,
> > I just have a gut feeling that it looks excessive.
>
> Note that I was talking about the wakeup done on ep->wq not ep->poll_wait.
> The path that I'm concerned about is let's say that there are N events
> queued on the ready list. A thread that was woken up in epoll_wait may
> decide to only process say N/2 of then. Then it will call wakeup on ep->wq
> and this will wakeup another thread to process the remaining N/2. Without
> the wakeup, the original thread isn't going to process the events until
> it finishes with the original N/2 and gets back to epoll_wait(). So I'm not
> sure how important that path is but I wanted to at least note the change
> here would impact that behavior.
>
> Thanks,
>
> -Jason
>
>
> >
> >> So I think removing all that even for the
> >> depth 0 case is going to change some behavior here. So perhaps, it
> >> should be removed for all depths except for 0? And if so, it may be
> >> better to make 2 patches here to separate these changes.
> >>
> >> For the nested wakeups, I agree that the extra wakeups seem unnecessary
> >> and it may make sense to remove them for all depths. I don't think the
> >> nested epoll semantics are particularly well spelled out, and afaict,
> >> nested epoll() has behaved this way for quite some time. And the current
> >> behavior is not bad in the way that a missing wakeup or false negative
> >> would be.
> >
> > That's 100% true! For edge mode extra wake up is not a bug, not optimal
> > for userspace - yes, but that can't lead to any lost wakeups.
> >
> > --
> > Roman
> >

I tried to remove the whole chunk of code that Roman said, and it
seems that there
are no obvious problems with the two test programs below:

Test case 1:
           t0
            |
           e0
            |
           e1 (et)
            |
           s0 (lt)

When s0 is readable, the thread 0 can only read once event from e0.

#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[2];
    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;

    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;

    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;

    close(efd[1]);
    close(efd[0]);
    close(sfd[0]);
    close(sfd[1]);

    printf("PASS\n");
    return 0;

out:
    printf("FAIL\n");
    return -1;
}

Test case 2:
           t0  t1
            \   /
             e0
            /   \
    (et) e1   e2 (et)
           |      |
     (lt) s0    s2 (lt)

When s0 and s2 are readable, both thread 0 and thread 1 can read an
event from e0.

#include <stdio.h>
#include <unistd.h>
#include <pthread.h>
#include <sys/epoll.h>
#include <sys/socket.h>

static int efd[3];
static int sfd[4];
static int count;

static void *
thread_handler(void *data)
{
    struct epoll_event e;

    if (epoll_wait(efd[0], &e, 1, -1) == 1)
        count++;

    return NULL;
}

static void *
emit_handler(void *data)
{
    usleep (100000);

    write(sfd[1], "w", 1);
    write(sfd[3], "w", 1);

    return NULL;
}

int main(int argc, char *argv[])
{
    struct epoll_event e;
    pthread_t tw, te;

    if (socketpair(AF_UNIX, SOCK_STREAM, 0, &sfd[0]) < 0)
        goto out;

    if (socketpair(AF_UNIX, SOCK_STREAM, 0, &sfd[2]) < 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[1], EPOLL_CTL_ADD, sfd[0], &e) < 0)
        goto out;

    e.events = EPOLLIN;
    if (epoll_ctl(efd[2], EPOLL_CTL_ADD, sfd[2], &e) < 0)
        goto out;

    e.events = EPOLLIN | EPOLLET;
    if (epoll_ctl(efd[0], EPOLL_CTL_ADD, efd[1], &e) < 0)
        goto out;

    e.events = EPOLLIN | EPOLLET;
    if (epoll_ctl(efd[0], EPOLL_CTL_ADD, efd[2], &e) < 0)
        goto out;

    if (pthread_create(&tw, NULL, thread_handler, NULL) < 0)
        goto out;

    if (pthread_create(&te, NULL, emit_handler, NULL) < 0)
        goto out;

    if (epoll_wait(efd[0], &e, 1, -1) == 1)
        count++;

    if (pthread_join(tw, NULL) < 0)
        goto out;

    if (count != 2)
        goto out;

    close(efd[0]);
    close(efd[1]);
    close(efd[2]);
    close(sfd[0]);
    close(sfd[1]);
    close(sfd[2]);
    close(sfd[3]);

    printf ("PASS\n");
    return 0;

out:
    printf("FAIL\n");
    return -1;
}

t0: thread0
t1: thread1
e0: epoll0 (efd[0])
e1: epoll1 (efd[1])
e2: epoll2 (efd[2])
s0: socket0 (sfd[0])
s2: socket2 (sfd[2])

Is it possible to prove that this modification is correct, or any
other corner cases are missing?

-- 
Best regards!
Hev
https://hev.cc