On Mon, 5 Apr 2021 16:10:25 -0700 Davidlohr Bueso <dave@xxxxxxxxxxxx> wrote: > 339ddb53d373 (fs/epoll: remove unnecessary wakeups of nested epoll) changed > the userspace visible behavior of exclusive waiters blocked on a common > epoll descriptor upon a single event becoming ready. Previously, all tasks > doing epoll_wait would awake, and now only one is awoken, potentially causing > missed wakeups on applications that rely on this behavior, such as Apache Qpid. > > While the aforementioned commit aims at having only a wakeup single path in > ep_poll_callback (with the exceptions of epoll_ctl cases), we need to restore > the wakeup in what was the old ep_scan_ready_list() such that the next thread > can be awoken, in a cascading style, after the waker's corresponding ep_send_events(). > Tricky. 339ddb53d373 was merged in December 2019. So do we backport this fix? Could any userspace code be depending upon the post-339ddb53d373 behaviour? Or do we just leave the post-339ddb53d373 code as-is? Presumably the issue is very rarely encountered, and changeing it back has its own risks.
