[PATCH 5.4 0/2] Fix epoll issue in 5.4 kernels

[Rishabh Bhatnagar]

Hi Greg
After upgrading to 5.4.211 we were started seeing some nodes getting
stuck in our Kubernetes cluster. All nodes are running this kernel
version. After taking a closer look it seems that runc was command getting
stuck. Looking at the stack it appears the thread is stuck in epoll wait for
sometime. 
[<0>] do_syscall_64+0x48/0xf0
[<0>] entry_SYSCALL_64_after_hwframe+0x5c/0xc1
[<0>] ep_poll+0x48d/0x4e0
[<0>] do_epoll_wait+0xab/0xc0
[<0>] __x64_sys_epoll_pwait+0x4d/0xa0
[<0>] do_syscall_64+0x48/0xf0
[<0>] entry_SYSCALL_64_after_hwframe+0x5c/0xc1
[<0>] futex_wait_queue_me+0xb6/0x110
[<0>] futex_wait+0xe2/0x260
[<0>] do_futex+0x372/0x4f0
[<0>] __x64_sys_futex+0x134/0x180
[<0>] do_syscall_64+0x48/0xf0
[<0>] entry_SYSCALL_64_after_hwframe+0x5c/0xc1

I noticed there are other discussions going on as well
regarding this.
https://lore.kernel.org/all/Y1pY2n6E1Xa58MXv@xxxxxxxxx/
Reverting the below patch does fix the issue:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=linux-5.4.y&id=cf2db24ec4b8e9d399005ececd6f6336916ab6fc
We don't see this issue in latest upstream kernel or even latest 5.10
stable tree. Looking at the patches that went in for 5.10 stable there's
one that stands out that seems to be missing in 5.4.
289caf5d8f6c61c6d2b7fd752a7f483cd153f182 (epoll: check for events when removing
a timed out thread from the wait queue)

Backporting this patch to 5.4 we don't see the hangups anymore. Looks like
this patch fixes time out scenarios which might cause missed wake ups.
The other patch in the patch series also fixes a race and is needed for
the second patch to apply.

Roman Penyaev (1):
  epoll: call final ep_events_available() check under the lock

Soheil Hassas Yeganeh (1):
  epoll: check for events when removing a timed out thread from the wait
    queue

 fs/eventpoll.c | 68 ++++++++++++++++++++++++++++++--------------------
 1 file changed, 41 insertions(+), 27 deletions(-)

-- 
2.37.1