cachefiles: make on-demand read killable

Replacing wait_for_completion() with wait_for_completion_killable() in
cachefiles_ondemand_send_req() allows us to kill processes that might
trigger a hunk_task if the daemon is abnormal.

But now only CACHEFILES_OP_READ is killable, because OP_CLOSE and OP_OPEN
is initiated from kworker context and the signal is prohibited in these
kworker.

Note that when the req in xas changes, i.e. xas_load(&xas) != req, it
means that a process will complete the current request soon, so wait
again for the request to be completed.

In addition, add the cachefiles_ondemand_finish_req() helper function to
simplify the code.
Suggested-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Link: https://lore.kernel.org/r/20240522114308.2402121-13-libaokun@huaweicloud.comAcked-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Jia Zhu <zhujia.zj@bytedance.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/cachefiles/ondemand.c

@@ -380,6 +380,20 @@ static struct cachefiles_req *cachefiles_ondemand_select_req(struct xa_state *xa
 	return NULL;
 }
 
+static inline bool cachefiles_ondemand_finish_req(struct cachefiles_req *req,
+						  struct xa_state *xas, int err)
+{
+	if (unlikely(!xas || !req))
+		return false;
+
+	if (xa_cmpxchg(xas->xa, xas->xa_index, req, NULL, 0) != req)
+		return false;
+
+	req->error = err;
+	complete(&req->done);
+	return true;
+}
+
 ssize_t cachefiles_ondemand_daemon_read(struct cachefiles_cache *cache,
 					char __user *_buffer, size_t buflen)
 {
@@ -443,16 +457,8 @@ ssize_t cachefiles_ondemand_daemon_read(struct cachefiles_cache *cache,
 out:
 	cachefiles_put_object(req->object, cachefiles_obj_put_read_req);
 	/* Remove error request and CLOSE request has no reply */
-	if (ret || msg->opcode == CACHEFILES_OP_CLOSE) {
-		xas_reset(&xas);
-		xas_lock(&xas);
-		if (xas_load(&xas) == req) {
-			req->error = ret;
-			complete(&req->done);
-			xas_store(&xas, NULL);
-		}
-		xas_unlock(&xas);
-	}
+	if (ret || msg->opcode == CACHEFILES_OP_CLOSE)
+		cachefiles_ondemand_finish_req(req, &xas, ret);
 	cachefiles_req_put(req);
 	return ret ? ret : n;
 }
@@ -544,8 +550,18 @@ static int cachefiles_ondemand_send_req(struct cachefiles_object *object,
 		goto out;
 
 	wake_up_all(&cache->daemon_pollwq);
-	wait_for_completion(&req->done);
-	ret = req->error;
+wait:
+	ret = wait_for_completion_killable(&req->done);
+	if (!ret) {
+		ret = req->error;
+	} else {
+		ret = -EINTR;
+		if (!cachefiles_ondemand_finish_req(req, &xas, ret)) {
+			/* Someone will complete it soon. */
+			cpu_relax();
+			goto wait;
+		}
+	}
 	cachefiles_req_put(req);
 	return ret;
 out: