io_uring: allow O_NONBLOCK async retry

We can assume that O_NONBLOCK is always honored, even if we don't
have a ->read/write_iter() for the file type. Also unify the read/write
checking for allowing async punt, having the write side factoring in the
REQ_F_NOWAIT flag as well.

Cc: stable@vger.kernel.org
Fixes: 490e8967 ("io_uring: only force async punt if poll based retry can't handle it")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -2061,6 +2061,10 @@ static bool io_file_supports_async(struct file *file, int rw)
 	if (S_ISREG(mode) && file->f_op != &io_uring_fops)
 		return true;
 
+	/* any ->read/write should understand O_NONBLOCK */
+	if (file->f_flags & O_NONBLOCK)
+		return true;
+
 	if (!(file->f_mode & FMODE_NOWAIT))
 		return false;
 
@@ -2103,8 +2107,7 @@ static int io_prep_rw(struct io_kiocb *req, const struct io_uring_sqe *sqe,
 		kiocb->ki_ioprio = get_current_ioprio();
 
 	/* don't allow async punt if RWF_NOWAIT was requested */
-	if ((kiocb->ki_flags & IOCB_NOWAIT) ||
-	    (req->file->f_flags & O_NONBLOCK))
+	if (kiocb->ki_flags & IOCB_NOWAIT)
 		req->flags |= REQ_F_NOWAIT;
 
 	if (force_nonblock)
@@ -2745,7 +2748,8 @@ static int io_write(struct io_kiocb *req, bool force_nonblock)
 			if (ret)
 				goto out_free;
 			/* any defer here is final, must blocking retry */
-			if (!file_can_poll(req->file))
+			if (!(req->flags & REQ_F_NOWAIT) &&
+			    !file_can_poll(req->file))
 				req->flags |= REQ_F_MUST_PUNT;
 			return -EAGAIN;
 		}