io_uring: refactor io_get_sqe()

The line of io_get_sqe() evaluating @head consists of too many
operations including READ_ONCE(), it's not convenient for probing.
Refactor it also improving readability.
Signed-off-by: NPavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/866ad6e4ef4851c7c61f6b0e08dbd0a8d1abce84.1623709150.git.asml.silence@gmail.comSigned-off-by: NJens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -6685,8 +6685,8 @@ static void io_commit_sqring(struct io_ring_ctx *ctx)
  */
 static const struct io_uring_sqe *io_get_sqe(struct io_ring_ctx *ctx)
 {
-	u32 *sq_array = ctx->sq_array;
 	unsigned head, mask = ctx->sq_entries - 1;
+	unsigned sq_idx = ctx->cached_sq_head++ & mask;
 
 	/*
 	 * The cached sq head (or cq tail) serves two purposes:
@@ -6696,7 +6696,7 @@ static const struct io_uring_sqe *io_get_sqe(struct io_ring_ctx *ctx)
 	 * 2) allows the kernel side to track the head on its own, even
 	 *    though the application is the one updating it.
 	 */
-	head = READ_ONCE(sq_array[ctx->cached_sq_head++ & mask]);
+	head = READ_ONCE(ctx->sq_array[sq_idx]);
 	if (likely(head < ctx->sq_entries))
 		return &ctx->sq_sqes[head];