kselftest: introduce new epoll test case

Patch series "fs/epoll: restore user-visible behavior upon event ready".

This series tries to address a change in user visible behavior, reported
in https://bugzilla.kernel.org/show_bug.cgi?id=208943.

Epoll does not report an event to all the threads running epoll_wait()
on the same epoll descriptor. Unsurprisingly, this was bisected back to
339ddb53d373 (fs/epoll: remove unnecessary wakeups of nested epoll), which
has had various problems in the past, beyond only nested epoll usage.

This patch (of 2):

This incorporates the testcase originally reported in:

https://bugzilla.kernel.org/show_bug.cgi?id=208943

Which ensures an event is reported to all threads blocked on the same
epoll descriptor, otherwise only a single thread will receive the wakeup
once the event become ready.

Link: https://lkml.kernel.org/r/20210405231025.33829-1-dave@stgolabs.net
Link: https://lkml.kernel.org/r/20210405231025.33829-2-dave@stgolabs.net
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Cc: Jason Baron <jbaron@akamai.com>
Cc: Roman Penyaev <rpenyaev@suse.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

selftests/filesystems: expand epoll with epoll_pwait2

Code coverage for the epoll_pwait2 syscall.

epoll62: Repeat basic test epoll1, but exercising the new syscall.
epoll63: Pass a timespec and exercise the timeout wakeup path.

Link: https://lkml.kernel.org/r/20201121144401.3727659-5-willemdebruijn.kernel@gmail.com
Signed-off-by: Willem de Bruijn <willemb@google.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

epoll: add a selftest for epoll timeout race

Add a test case to ensure an event is observed by at least one poller
when an epoll timeout is used.

Signed-off-by: Guantao Liu <guantaol@google.com>
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Khazhismel Kumykov <khazhy@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Link: https://lkml.kernel.org/r/20201028180202.952079-2-soheil.kdev@gmail.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kselftests: introduce new epoll60 testcase for catching lost wakeups

This test case catches lost wake up introduced by commit 339ddb53d373
("fs/epoll: remove unnecessary wakeups of nested epoll")

The test is simple: we have 10 threads and 10 event fds. Each thread
can harvest only 1 event. 1 producer fires all 10 events at once and
waits that all 10 events will be observed by 10 threads.

In case of lost wakeup epoll_wait() will timeout and 0 will be returned.

Test case catches two sort of problems: forgotten wakeup on event, which
hits the ->ovflist list, this problem was fixed by:

5a2513239750 ("eventpoll: fix missing wakeup for ovflist in ep_poll_callback")

the other problem is when several sequential events hit the same waiting
thread, thus other waiters get no wakeups. Problem is fixed in the
following patch.

Signed-off-by: Roman Penyaev <rpenyaev@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Khazhismel Kumykov <khazhy@google.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Heiher <r@hev.cc>
Cc: Jason Baron <jbaron@akamai.com>
Link: http://lkml.kernel.org/r/20200430130326.1368509-1-rpenyaev@suse.de
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kselftest: introduce new epoll test case

This testcase repeats epollbug.c from the bug:

https://bugzilla.kernel.org/show_bug.cgi?id=205933

What it tests? It tests the race between epoll_ctl() and epoll_wait().
New event mask passed to epoll_ctl() triggers wake up, which can be missed
because of the bug described in the link. Reproduction is 100%, so easy
to fix. Kudos, Max, for wonderful test case.

Signed-off-by: Roman Penyaev <rpenyaev@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Max Neunhoeffer <max@arangodb.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Christopher Kohlhoff <chris.kohlhoff@clearpool.io>
Cc: Davidlohr Bueso <dbueso@suse.de>
Cc: Jason Baron <jbaron@akamai.com>
Cc: Jes Sorensen <jes.sorensen@gmail.com>
Link: http://lkml.kernel.org/r/20200214170211.561524-2-rpenyaev@suse.de
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

selftests: add epoll selftests

This adds the promised selftest for epoll. It will verify the wakeups
of epoll. Including leaf and nested mode, epoll_wait() and poll() and
multi-threads.

Link: http://lkml.kernel.org/r/20191009121518.4027-1-r@hev.cc
Signed-off-by: hev <r@hev.cc>
Reviewed-by: Roman Penyaev <rpenyaev@suse.de>
Cc: Jason Baron <jbaron@akamai.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>