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, 9 insertions, 13 deletions

diff --git a/fs/pipe.c b/fs/pipe.c
index bb0840e234f3..39d6f431da83 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -509,22 +509,19 @@ static long pipe_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
         }
 }
 
-static struct wait_queue_head *
-pipe_get_poll_head(struct file *filp, __poll_t events)
-{
-        struct pipe_inode_info *pipe = filp->private_data;
-
-        return &pipe->wait;
-}
-
 /* No kernel lock held - fine */
-static __poll_t pipe_poll_mask(struct file *filp, __poll_t events)
+static __poll_t
+pipe_poll(struct file *filp, poll_table *wait)
 {
+        __poll_t mask;
         struct pipe_inode_info *pipe = filp->private_data;
-        int nrbufs = pipe->nrbufs;
-        __poll_t mask = 0;
+        int nrbufs;
+
+        poll_wait(filp, &pipe->wait, wait);
 
         /* Reading only -- no need for acquiring the semaphore.  */
+        nrbufs = pipe->nrbufs;
+        mask = 0;
         if (filp->f_mode & FMODE_READ) {
                 mask = (nrbufs > 0) ? EPOLLIN | EPOLLRDNORM : 0;
                 if (!pipe->writers && filp->f_version != pipe->w_counter)
@@ -1023,8 +1020,7 @@ const struct file_operations pipefifo_fops = {
         .llseek         = no_llseek,
         .read_iter      = pipe_read,
         .write_iter     = pipe_write,
-        .get_poll_head  = pipe_get_poll_head,
-        .poll_mask      = pipe_poll_mask,
+        .poll           = pipe_poll,
         .unlocked_ioctl = pipe_ioctl,
         .release        = pipe_release,
         .fasync         = pipe_fasync,