On Fri, 31 Jan 2020 14:57:30 +0100, Max Neunhoeffer wrote:

Dear All,

I believe I have found a bug in Linux 5.3 and 5.4 in epoll_wait/epoll_ctl
when an eventfd together with edge-triggered or the EPOLLONESHOT policy
is used. If an epoll_ctl call to rearm the eventfd happens approximately
at the same time as the epoll_wait goes to sleep, the event can be lost, 
even though proper protection through a mutex is employed.

The details together with two programs showing the problem can be found
here:

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

Older kernels seem not to have this problem, although I did not test all
versions. I know that 4.15 and 5.0 do not show the problem.

Note that this method of using epoll_wait/eventfd is used by
boost::asio to wake up event loops in case a new completion handler
is posted to an io_service, so this is probably relevant for many
applications.

Any help with this would be appreciated.

Could be networking related but let's CC FS folks just in case.

Would you be able to perform bisection to narrow down the search 
for a buggy change?