io_uring: silence variable ‘prev’ set but not used warning

If io_uring.o is built with W=1, it triggers a warning:

io_uring/io_uring.c: In function ‘__io_submit_flush_completions’:
io_uring/io_uring.c:1502:40: warning: variable ‘prev’ set but not used [-Wunused-but-set-variable]
 1502 |         struct io_wq_work_node *node, *prev;
      |                                        ^~~~

which is due to the wq_list_for_each() iterator always keeping a 'prev'
variable. Most users need this to remove an entry from a list, for
example, but __io_submit_flush_completions() never does that.

Add a basic helper that doesn't track prev instead, and use that in
that function.
Reported-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/io_uring.c

@@ -1499,14 +1499,14 @@ void io_free_batch_list(struct io_ring_ctx *ctx, struct io_wq_work_node *node)
 static void __io_submit_flush_completions(struct io_ring_ctx *ctx)
 	__must_hold(&ctx->uring_lock)
 {
-	struct io_wq_work_node *node, *prev;
 	struct io_submit_state *state = &ctx->submit_state;
+	struct io_wq_work_node *node;
 
 	__io_cq_lock(ctx);
 	/* must come first to preserve CQE ordering in failure cases */
 	if (state->cqes_count)
 		__io_flush_post_cqes(ctx);
-	wq_list_for_each(node, prev, &state->compl_reqs) {
+	__wq_list_for_each(node, &state->compl_reqs) {
 		struct io_kiocb *req = container_of(node, struct io_kiocb,
 					    comp_list);
 

io_uring/slist.h

@@ -3,6 +3,9 @@
 
 #include <linux/io_uring_types.h>
 
+#define __wq_list_for_each(pos, head)				\
+	for (pos = (head)->first; pos; pos = (pos)->next)
+
 #define wq_list_for_each(pos, prv, head)			\
 	for (pos = (head)->first, prv = NULL; pos; prv = pos, pos = (pos)->next)
 
@@ -113,4 +116,4 @@ static inline struct io_wq_work *wq_next_work(struct io_wq_work *work)
 	return container_of(work->list.next, struct io_wq_work, list);
 }
 
-#endif // INTERNAL_IO_SLIST_H
+#endif // INTERNAL_IO_SLIST_H
\ No newline at end of file