Re: [PATCH] io_uring: allow async accept on O_NONBLOCK sockets

[Jens Axboe <axboe@kernel.dk>]

On 3/24/22 8:42 AM, Jens Axboe wrote:
> On 3/24/22 8:34 AM, Dylan Yudaken wrote:
>> Do not set REQ_F_NOWAIT if the socket is non blocking. When enabled this
>> causes the accept to immediately post a CQE with EAGAIN, which means you
>> cannot perform an accept SQE on a NONBLOCK socket asynchronously.
>>
>> By removing the flag if there is no pending accept then poll is armed as
>> usual and when a connection comes in the CQE is posted.
>>
>> note: If multiple accepts are queued up, then when a single connection
>> comes in they all complete, one with the connection, and the remaining
>> with EAGAIN. This could be improved in the future but will require a lot
>> of io_uring changes.
> 
> Not true - all you'd need to do is have behavior similar to
> EPOLLEXCLUSIVE, which we already support for separate poll. Could be
> done for internal poll quite easily, and _probably_ makes sense to do by
> default for most cases in fact.

Quick wire-up below. Not tested at all, but really should basically as
simple as this.

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 4d98cc820a5c..8dfacb476726 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -967,6 +967,7 @@ struct io_op_def {
 	/* set if opcode supports polled "wait" */
 	unsigned		pollin : 1;
 	unsigned		pollout : 1;
+	unsigned		poll_exclusive : 1;
 	/* op supports buffer selection */
 	unsigned		buffer_select : 1;
 	/* do prep async if is going to be punted */
@@ -1061,6 +1062,7 @@ static const struct io_op_def io_op_defs[] = {
 		.needs_file		= 1,
 		.unbound_nonreg_file	= 1,
 		.pollin			= 1,
+		.poll_exclusive		= 1,
 	},
 	[IORING_OP_ASYNC_CANCEL] = {
 		.audit_skip		= 1,
@@ -6293,6 +6295,8 @@ static int io_arm_poll_handler(struct io_kiocb *req, unsigned issue_flags)
 	} else {
 		mask |= POLLOUT | POLLWRNORM;
 	}
+	if (def->poll_exclusive)
+		mask |= EPOLLEXCLUSIVE;
 
 	if (!(issue_flags & IO_URING_F_UNLOCKED) &&
 	    !list_empty(&ctx->apoll_cache)) {

-- 
Jens Axboe