io_uring: fix io_free_batch_list races

[  158.514382] WARNING: CPU: 5 PID: 15251 at fs/io_uring.c:1141 io_free_batch_list+0x269/0x360
[  158.514426] RIP: 0010:io_free_batch_list+0x269/0x360
[  158.514437] Call Trace:
[  158.514440]  __io_submit_flush_completions+0xde/0x180
[  158.514444]  tctx_task_work+0x14a/0x220
[  158.514447]  task_work_run+0x64/0xa0
[  158.514448]  __do_sys_io_uring_enter+0x7c/0x970
[  158.514450]  __x64_sys_io_uring_enter+0x22/0x30
[  158.514451]  do_syscall_64+0x43/0x90
[  158.514453]  entry_SYSCALL_64_after_hwframe+0x44/0xae

We should not touch request internals including req->comp_list.next
after putting our ref if it's not final, e.g. we can start freeing
requests from the free cache.

Fixed: 62ca9cb93e7f8 ("io_uring: optimise io_free_batch_list()")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/b1f4df38fbb8f111f52911a02fd418d0283a4e6f.1634047298.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -2266,8 +2266,9 @@ static void io_free_batch_list(struct io_ring_ctx *ctx,
 		struct io_kiocb *req = container_of(node, struct io_kiocb,
 						    comp_list);
 
-		if (!req_ref_put_and_test(req)) {
+		if (unlikely(req->flags & REQ_F_REFCOUNT)) {
 			node = req->comp_list.next;
+			if (!req_ref_put_and_test(req))
 				continue;
 		}