io_uring: improve io_get_sqe

Return an SQE from io_get_sqe() as a parameter and use the return value
to determine if it failed or not. This enables the compiler to compile out
the sqe NULL check when we know that the return SQE is valid.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/9cceb11329240ea097dffef6bf0a675bca14cf42.1674484266.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/io_uring.c

@@ -2370,7 +2370,7 @@ static void io_commit_sqring(struct io_ring_ctx *ctx)
  * used, it's important that those reads are done through READ_ONCE() to
  * prevent a re-load down the line.
  */
-static const struct io_uring_sqe *io_get_sqe(struct io_ring_ctx *ctx)
+static const bool io_get_sqe(struct io_ring_ctx *ctx, const struct io_uring_sqe **sqe)
 {
 	unsigned head, mask = ctx->sq_entries - 1;
 	unsigned sq_idx = ctx->cached_sq_head++ & mask;
@@ -2388,14 +2388,15 @@ static const struct io_uring_sqe *io_get_sqe(struct io_ring_ctx *ctx)
 		/* double index for 128-byte SQEs, twice as long */
 		if (ctx->flags & IORING_SETUP_SQE128)
 			head <<= 1;
-		return &ctx->sq_sqes[head];
+		*sqe = &ctx->sq_sqes[head];
+		return true;
 	}
 
 	/* drop invalid entries */
 	ctx->cq_extra--;
 	WRITE_ONCE(ctx->rings->sq_dropped,
 		   READ_ONCE(ctx->rings->sq_dropped) + 1);
-	return NULL;
+	return false;
 }
 
 int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr)
@@ -2419,8 +2420,7 @@ int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr)
 		if (unlikely(!io_alloc_req_refill(ctx)))
 			break;
 		req = io_alloc_req(ctx);
-		sqe = io_get_sqe(ctx);
-		if (unlikely(!sqe)) {
+		if (unlikely(!io_get_sqe(ctx, &sqe))) {
 			io_req_add_to_cache(req, ctx);
 			break;
 		}