io_uring: use release-acquire ordering for IORING_SETUP_R_DISABLED

io_uring: use release-acquire ordering for IORING_SETUP_R_DISABLED

io_uring_enter(), __io_msg_ring_data(), and io_msg_send_fd() read
ctx->flags and ctx->submitter_task without holding the ctx's uring_lock.
This means they may race with the assignment to ctx->submitter_task and
the clearing of IORING_SETUP_R_DISABLED from ctx->flags in
io_register_enable_rings(). Ensure the correct ordering of the
ctx->flags and ctx->submitter_task memory accesses by storing to
ctx->flags using release ordering and loading it using acquire ordering.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Fixes: 4add705e4eeb ("io_uring: remove io_register_submitter")
Reviewed-by: Joanne Koong <joannelkoong@gmail.com>
Reviewed-by: Gabriel Krisman Bertazi <krisman@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>