io_uring/rw: ensure poll based multishot read retries appropriately

io_read_mshot() always relies on poll triggering retries, and this works
fine as long as we do a retry per size of the buffer being read. The
buffer size is given by the size of the buffer(s) in the given buffer
group ID.

But if we're reading less than what is available, then we don't always
get to read everything that is available. For example, if the buffers
available are 32 bytes and we have 64 bytes to read, then we'll
correctly read the first 32 bytes and then wait for another poll trigger
before we attempt the next read. This next poll trigger may never
happen, in which case we just sit forever and never make progress, or it
may trigger at some point in the future, and now we're just delivering
the available data much later than we should have.

io_read_mshot() could do retries itself, but that is wasteful as we'll
be going through all of __io_read() again, and most likely in vain.
Rather than do that, bump our poll reference count and have
io_poll_check_events() do one more loop and check with vfs_poll() if we
have more data to read. If we do, io_read_mshot() will get invoked again
directly and we'll read the next chunk.

io_poll_multishot_retry() must only get called from inside
io_poll_issue(), which is our multishot retry handler, as we know we
already "own" the request at this point.

Cc: stable@vger.kernel.org
Link: https://github.com/axboe/liburing/issues/1041
Fixes: fc68fcda0491 ("io_uring/rw: add support for IORING_OP_READ_MULTISHOT")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring: avoid indirect function calls for the hottest task_work

We use task_work for a variety of reasons, but doing completions or
triggering rety after poll are by far the hottest two. Use the indirect
funtion call wrappers to avoid the indirect function call if
CONFIG_RETPOLINE is set.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/poll: allow some retries for poll triggering spuriously

If we get woken spuriously when polling and fail the operation with
-EAGAIN again, then we generally only allow polling again if data
had been transferred at some point. This is indicated with
REQ_F_PARTIAL_IO. However, if the spurious poll triggers when the socket
was originally empty, then we haven't transferred data yet and we will
fail the poll re-arm. This either punts the socket to io-wq if it's
blocking, or it fails the request with -EAGAIN if not. Neither condition
is desirable, as the former will slow things down, while the latter
will make the application confused.

We want to ensure that a repeated poll trigger doesn't lead to infinite
work making no progress, that's what the REQ_F_PARTIAL_IO check was
for. But it doesn't protect against a loop post the first receive, and
it's unnecessarily strict if we started out with an empty socket.

Add a somewhat random retry count, just to put an upper limit on the
potential number of retries that will be done. This should be high enough
that we won't really hit it in practice, unless something needs to be
aborted anyway.

Cc: stable@vger.kernel.org # v5.10+
Link: https://github.com/axboe/liburing/issues/364
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring: add abstraction around apoll cache

In preparation for adding limits, and one more user, abstract out the
core bits of the allocation+free cache.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring: move apoll cache to poll.c

This is where it's used, move the flush handler in there.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring: move a few private types to local headers

Commit 3a3d47fa9cfd ("io_uring: make io_uring_types.h public") moved
a bunch of io_uring types to a kernel wide header, so we could make
tracing a bit saner rather than pass in a ton of arguments.

However, there are a few types in there that are not really needed to
be system wide. Move the cancel data and mapped buffers back to the
appropriate io_uring local headers.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring: propagate locking state to poll cancel

Poll cancellation will be soon need to grab ->uring_lock inside, pass
the locking state, i.e. issue_flags, inside the cancellation functions.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/b86781d047727c07163443b57551a3fa57c7c5e1.1655371007.git.asml.silence@gmail.com
Reviewed-by: Hao Xu <howeyxu@tencent.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring: move poll handling into its own file

Add a io_poll_issue() rather than export the general task_work locking
and io_issue_sqe(), and put the io_op_defs definition and structure into
a separate header file so that poll can use it.

Signed-off-by: Jens Axboe <axboe@kernel.dk>