[git pull] epoll rework

[Al Viro]

	Deal with epoll loop check/removal races sanely (among other things).
Solution merged last cycle (pinning a bunch of struct file instances) had
been forced by the wrong data structures; untangling that takes a bunch
of preparations, but it's worth doing - control flow in there is ridiculously
overcomplicated.  Memory footprint has also gone down, while we are at it.
This is not all I want to do in the area, but since I didn't get around to
posting the followups they'll have to wait for the next cycle.

The following changes since commit 3650b228f83adda7e5ee532e2b90429c03f7b9ec:

  Linux 5.10-rc1 (2020-10-25 15:14:11 -0700)

are available in the git repository at:

  git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs.git work.epoll

for you to fetch changes up to 319c15174757aaedacc89a6e55c965416f130e64:

  epoll: take epitem list out of struct file (2020-10-25 20:02:08 -0400)

----------------------------------------------------------------
Al Viro (27):
      epoll: switch epitem->pwqlist to single-linked list
      epoll: get rid of epitem->nwait
      untangling ep_call_nested(): get rid of useless arguments
      untangling ep_call_nested(): it's all serialized on epmutex.
      untangling ep_call_nested(): take pushing cookie into a helper
      untangling ep_call_nested(): move push/pop of cookie into the callbacks
      untangling ep_call_nested(): and there was much rejoicing
      reverse_path_check_proc(): sane arguments
      reverse_path_check_proc(): don't bother with cookies
      clean reverse_path_check_proc() a bit
      ep_loop_check_proc(): lift pushing the cookie into callers
      get rid of ep_push_nested()
      ep_loop_check_proc(): saner calling conventions
      ep_scan_ready_list(): prepare to splitup
      lift the calls of ep_read_events_proc() into the callers
      lift the calls of ep_send_events_proc() into the callers
      ep_send_events_proc(): fold into the caller
      lift locking/unlocking ep->mtx out of ep_{start,done}_scan()
      ep_insert(): don't open-code ep_remove() on failure exits
      ep_insert(): we only need tep->mtx around the insertion itself
      take the common part of ep_eventpoll_poll() and ep_item_poll() into helper
      fold ep_read_events_proc() into the only caller
      ep_insert(): move creation of wakeup source past the fl_ep_links insertion
      convert ->f_ep_links/->fllink to hlist
      lift rcu_read_lock() into reverse_path_check()
      epoll: massage the check list insertion
      epoll: take epitem list out of struct file

 fs/eventpoll.c            | 717 +++++++++++++++++++---------------------------
 fs/file_table.c           |   1 -
 include/linux/eventpoll.h |  11 +-
 include/linux/fs.h        |   5 +-
 4 files changed, 305 insertions(+), 429 deletions(-)