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, 43 deletions

diff --git a/net/socket.c b/net/socket.c
index 8a109012608a..a564c6ed19d5 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -117,10 +117,8 @@ static ssize_t sock_write_iter(struct kiocb *iocb, struct iov_iter *from);
 static int sock_mmap(struct file *file, struct vm_area_struct *vma);
 
 static int sock_close(struct inode *inode, struct file *file);
-static struct wait_queue_head *sock_get_poll_head(struct file *file,
-                __poll_t events);
-static __poll_t sock_poll_mask(struct file *file, __poll_t);
-static __poll_t sock_poll(struct file *file, struct poll_table_struct *wait);
+static __poll_t sock_poll(struct file *file,
+                              struct poll_table_struct *wait);
 static long sock_ioctl(struct file *file, unsigned int cmd, unsigned long arg);
 #ifdef CONFIG_COMPAT
 static long compat_sock_ioctl(struct file *file,
@@ -143,8 +141,6 @@ static const struct file_operations socket_file_ops = {
         .llseek =       no_llseek,
         .read_iter =    sock_read_iter,
         .write_iter =   sock_write_iter,
-        .get_poll_head = sock_get_poll_head,
-        .poll_mask =    sock_poll_mask,
         .poll =         sock_poll,
         .unlocked_ioctl = sock_ioctl,
 #ifdef CONFIG_COMPAT
@@ -1130,48 +1126,14 @@ out_release:
 }
 EXPORT_SYMBOL(sock_create_lite);
 
-static struct wait_queue_head *sock_get_poll_head(struct file *file,
-                __poll_t events)
-{
-        struct socket *sock = file->private_data;
-
-        if (!sock->ops->poll_mask)
-                return NULL;
-        sock_poll_busy_loop(sock, events);
-        return sk_sleep(sock->sk);
-}
-
-static __poll_t sock_poll_mask(struct file *file, __poll_t events)
-{
-        struct socket *sock = file->private_data;
-
-        /*
-         * We need to be sure we are in sync with the socket flags modification.
-         *
-         * This memory barrier is paired in the wq_has_sleeper.
-         */
-        smp_mb();
-
-        /* this socket can poll_ll so tell the system call */
-        return sock->ops->poll_mask(sock, events) |
-                (sk_can_busy_loop(sock->sk) ? POLL_BUSY_LOOP : 0);
-}
-
 /* No kernel lock held - perfect */
 static __poll_t sock_poll(struct file *file, poll_table *wait)
 {
         struct socket *sock = file->private_data;
-        __poll_t events = poll_requested_events(wait), mask = 0;
-
-        if (sock->ops->poll) {
-                sock_poll_busy_loop(sock, events);
-                mask = sock->ops->poll(file, sock, wait);
-        } else if (sock->ops->poll_mask) {
-                sock_poll_wait(file, sock_get_poll_head(file, events), wait);
-                mask = sock->ops->poll_mask(sock, events);
-        }
+        __poll_t events = poll_requested_events(wait);
 
-        return mask | sock_poll_busy_flag(sock);
+        sock_poll_busy_loop(sock, events);
+        return sock->ops->poll(file, sock, wait) | sock_poll_busy_flag(sock);
 }
 
 static int sock_mmap(struct file *file, struct vm_area_struct *vma)