SUNRPC: Cache deferral injection

Cache deferral injection stress-tests the cache deferral logic as
well as upper layer protocol deferred request handlers. This
facility is for developers and professional testers to ensure
coverage of the rqst deferral code paths. To date, we haven't
had an adequate way to ensure these code paths are covered
during testing, short of temporary code changes to force their
use.

A file called /sys/kernel/debug/fail_sunrpc/ignore-cache-wait
enables administrators to disable cache deferral injection while
allowing other types of sunrpc errors to be injected. The default
setting is that cache deferral injection is enabled (ignore=false).

To enable support for cache deferral injection,
CONFIG_FAULT_INJECTION, CONFIG_FAULT_INJECTION_DEBUG_FS, and
CONFIG_SUNRPC_DEBUG must all be set to "Y".

Signed-off-by: Chuck Lever <[e-mail]>

net/sunrpc/cache.c

···
 #include <linux/sunrpc/stats.h>
 #include <linux/sunrpc/rpc_pipe_fs.h>
 #include <trace/events/sunrpc.h>
+
 #include "netns.h"
+#include "fail.h"
 
 #define	 RPCDBG_FACILITY RPCDBG_CACHE
 
···
 		discard->revisit(discard, 1);
 }
 
+#if IS_ENABLED(CONFIG_FAIL_SUNRPC)
+static inline bool cache_defer_immediately(void)
+{
+	return !fail_sunrpc.ignore_cache_wait &&
+		should_fail(&fail_sunrpc.attr, 1);
+}
+#else
+static inline bool cache_defer_immediately(void)
+{
+	return false;
+}
+#endif
+
 /* Return true if and only if a deferred request is queued. */
 static bool cache_defer_req(struct cache_req *req, struct cache_head *item)
 {
 	struct cache_deferred_req *dreq;
 
-	if (req->thread_wait) {
+	if (!cache_defer_immediately()) {
 		cache_wait_req(req, item);
 		if (!test_bit(CACHE_PENDING, &item->flags))
 			return false;
 	}
+
 	dreq = req->defer(req);
 	if (dreq == NULL)
 		return false;

net/sunrpc/debugfs.c

···
 
 	debugfs_create_bool("ignore-server-disconnect", S_IFREG | 0600, dir,
 			    &fail_sunrpc.ignore_server_disconnect);
+
+	debugfs_create_bool("ignore-cache-wait", S_IFREG | 0600, dir,
+			    &fail_sunrpc.ignore_cache_wait);
 }
 #else
 static void fail_sunrpc_init(void)

net/sunrpc/fail.h

···
 	struct fault_attr	attr;
 
 	bool			ignore_client_disconnect;
-
 	bool			ignore_server_disconnect;
+	bool			ignore_cache_wait;
 };
 
 extern struct fail_sunrpc_attr fail_sunrpc;