io_uring: cmpxchg for poll arm refs release

[ upstream commit 2f3893437a4ebf2e892ca172e9e122841319d675 ]

Replace atomically substracting the ownership reference at the end of
arming a poll with a cmpxchg. We try to release ownership by setting 0
assuming that poll_refs didn't change while we were arming. If it did
change, we keep the ownership and use it to queue a tw, which is fully
capable to process all events and (even tolerates spurious wake ups).

It's a bit more elegant as we reduce races b/w setting the cancellation
flag and getting refs with this release, and with that we don't have to
worry about any kinds of underflows. It's not the fastest path for
polling. The performance difference b/w cmpxchg and atomic dec is
usually negligible and it's not the fastest path.

Cc: stable@vger.kernel.org
Fixes: aa43477b04025 ("io_uring: poll rework")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/0c95251624397ea6def568ff040cad2d7926fd51.1668963050.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 62e0d35..0e3fc80 100644 (file)
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -5650,7 +5650,6 @@ static int __io_arm_poll_handler(struct io_kiocb *req,
                                 struct io_poll_table *ipt, __poll_t mask)
 {
        struct io_ring_ctx *ctx = req->ctx;
-       int v;
 
        INIT_HLIST_NODE(&req->hash_node);
        io_init_poll_iocb(poll, mask, io_poll_wake);
@@ -5696,11 +5695,10 @@ static int __io_arm_poll_handler(struct io_kiocb *req,
        }
 
        /*
-        * Release ownership. If someone tried to queue a tw while it was
-        * locked, kick it off for them.
+        * Try to release ownership. If we see a change of state, e.g.
+        * poll was waken up, queue up a tw, it'll deal with it.
         */
-       v = atomic_dec_return(&req->poll_refs);
-       if (unlikely(v & IO_POLL_REF_MASK))
+       if (atomic_cmpxchg(&req->poll_refs, 1, 0) != 1)
                __io_poll_execute(req, 0);
        return 0;
 }