Revert changes to convert to ->poll_mask() and aio IOCB_CMD_POLL

The poll() changes were not well thought out, and completely
unexplained.  They also caused a huge performance regression, because
"->poll()" was no longer a trivial file operation that just called down
to the underlying file operations, but instead did at least two indirect
calls.

Indirect calls are sadly slow now with the Spectre mitigation, but the
performance problem could at least be largely mitigated by changing the
"->get_poll_head()" operation to just have a per-file-descriptor pointer
to the poll head instead.  That gets rid of one of the new indirections.

But that doesn't fix the new complexity that is completely unwarranted
for the regular case.  The (undocumented) reason for the poll() changes
was some alleged AIO poll race fixing, but we don't make the common case
slower and more complex for some uncommon special case, so this all
really needs way more explanations and most likely a fundamental
redesign.

[ This revert is a revert of about 30 different commits, not reverted
  individually because that would just be unnecessarily messy  - Linus ]

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 1 deletions

diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index d20f7addee19..ce620e878538 100644
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -7717,12 +7717,14 @@ out:
  * here, again, by modeling the current TCP/UDP code.  We don't have
  * a good way to test with it yet.
  */
-__poll_t sctp_poll_mask(struct socket *sock, __poll_t events)
+__poll_t sctp_poll(struct file *file, struct socket *sock, poll_table *wait)
 {
         struct sock *sk = sock->sk;
         struct sctp_sock *sp = sctp_sk(sk);
         __poll_t mask;
 
+        poll_wait(file, sk_sleep(sk), wait);
+
         sock_rps_record_flow(sk);
 
         /* A TCP-style listening socket becomes readable when the accept queue