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, 5 insertions, 10 deletions

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index ea4436f409fb..67db22fe99c5 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -922,18 +922,14 @@ static __poll_t ep_read_events_proc(struct eventpoll *ep, struct list_head *head
         return 0;
 }
 
-static struct wait_queue_head *ep_eventpoll_get_poll_head(struct file *file,
-                __poll_t eventmask)
-{
-        struct eventpoll *ep = file->private_data;
-        return &ep->poll_wait;
-}
-
-static __poll_t ep_eventpoll_poll_mask(struct file *file, __poll_t eventmask)
+static __poll_t ep_eventpoll_poll(struct file *file, poll_table *wait)
 {
         struct eventpoll *ep = file->private_data;
         int depth = 0;
 
+        /* Insert inside our poll wait queue */
+        poll_wait(file, &ep->poll_wait, wait);
+
         /*
          * Proceed to find out if wanted events are really available inside
          * the ready list.
@@ -972,8 +968,7 @@ static const struct file_operations eventpoll_fops = {
         .show_fdinfo    = ep_show_fdinfo,
 #endif
         .release        = ep_eventpoll_release,
-        .get_poll_head  = ep_eventpoll_get_poll_head,
-        .poll_mask      = ep_eventpoll_poll_mask,
+        .poll           = ep_eventpoll_poll,
         .llseek         = noop_llseek,
 };