examples: enable io_uring single-issuer mode

These example filesystems always send their reply from the per-queue
io-uring worker thread. The ones that run a helper thread use it only
for cache-invalidation notifications over /dev/fuse, not ring
submissions, so they too satisfy the io_uring_single_issuer contract.

Call fuse_set_conn_flag(conn, FUSE_CONN_FLAG_SINGLE_ISSUER) in their
init() handlers so they take the lock-free, registered-ring-fd fast
path. It is harmless when io-uring is not in use.

Signed-off-by: Bernd Schubert <bernd@bsbernd.com>

example/hello.c

@@ -55,7 +55,9 @@ static const struct fuse_opt option_spec[] = {
 static void *hello_init(struct fuse_conn_info *conn,
 			struct fuse_config *cfg)
 {
-	(void) conn;
+	/* Always replies inline on the io-uring worker thread */
+	fuse_set_conn_flag(conn, FUSE_CONN_FLAG_SINGLE_ISSUER);
+
 	cfg->kernel_cache = 1;
 
 	/* Test setting flags the old way */

example/hello_ll.c

@@ -61,6 +61,9 @@ static void hello_ll_init(void *userdata, struct fuse_conn_info *conn)
 {
 	(void)userdata;
 
+	/* Always replies inline on the io-uring worker thread */
+	fuse_set_conn_flag(conn, FUSE_CONN_FLAG_SINGLE_ISSUER);
+
 	/* Disable the receiving and processing of FUSE_INTERRUPT requests */
 	conn->no_interrupt = 1;
 

example/invalidate_path.c

@@ -72,7 +72,8 @@ static const struct fuse_opt option_spec[] = {
 
 static void *xmp_init(struct fuse_conn_info *conn, struct fuse_config *cfg)
 {
-	(void) conn;
+	fuse_set_conn_flag(conn, FUSE_CONN_FLAG_SINGLE_ISSUER);
+
 	cfg->entry_timeout = NO_TIMEOUT;
 	cfg->attr_timeout = NO_TIMEOUT;
 	cfg->negative_timeout = 0;

example/ioctl_ll.c

@@ -109,7 +109,9 @@ static int fioc_stat(fuse_ino_t ino, struct stat *stbuf)
 static void fioc_ll_init(void *userdata, struct fuse_conn_info *conn)
 {
 	(void)userdata;
-	(void)conn;
+
+	/* Always replies inline on the io-uring worker thread */
+	fuse_set_conn_flag(conn, FUSE_CONN_FLAG_SINGLE_ISSUER);
 }
 
 static void fioc_ll_getattr(fuse_req_t req, fuse_ino_t ino,

example/memfs_ll.cc

@@ -833,6 +833,10 @@ static void memfs_releasedir(fuse_req_t req, [[maybe_unused]] fuse_ino_t ino,
 static void memfs_init(void *userdata, struct fuse_conn_info *conn)
 {
 	(void)userdata;
+
+	/* Always replies inline on the io-uring worker thread */
+	fuse_set_conn_flag(conn, FUSE_CONN_FLAG_SINGLE_ISSUER);
+
 	if (memfs_cfg.writeback_cache)
 		fuse_set_feature_flag(conn, FUSE_CAP_WRITEBACK_CACHE);
 }

example/notify_inval_entry.c

@@ -152,6 +152,8 @@ static int tfs_stat(fuse_ino_t ino, struct stat *stbuf) {
 static void tfs_init(void *userdata, struct fuse_conn_info *conn) {
 	(void)userdata;
 
+	fuse_set_conn_flag(conn, FUSE_CONN_FLAG_SINGLE_ISSUER);
+
 	/* Disable the receiving and processing of FUSE_INTERRUPT requests */
 	conn->no_interrupt = 1;
 }

example/notify_inval_inode.c

@@ -126,6 +126,8 @@ static int tfs_stat(fuse_ino_t ino, struct stat *stbuf) {
 static void tfs_init(void *userdata, struct fuse_conn_info *conn) {
 	(void)userdata;
 
+	fuse_set_conn_flag(conn, FUSE_CONN_FLAG_SINGLE_ISSUER);
+
 	/* Disable the receiving and processing of FUSE_INTERRUPT requests */
 	conn->no_interrupt = 1;
 }

example/notify_prune.c

@@ -137,6 +137,8 @@ static void tfs_init(void *userdata, struct fuse_conn_info *conn)
 {
 	(void)userdata;
 
+	fuse_set_conn_flag(conn, FUSE_CONN_FLAG_SINGLE_ISSUER);
+
 	/* Disable the receiving and processing of FUSE_INTERRUPT requests */
 	conn->no_interrupt = 1;
 }

example/notify_store_retrieve.c

@@ -135,6 +135,8 @@ static int tfs_stat(fuse_ino_t ino, struct stat *stbuf) {
 static void tfs_init(void *userdata, struct fuse_conn_info *conn) {
 	(void)userdata;
 
+	fuse_set_conn_flag(conn, FUSE_CONN_FLAG_SINGLE_ISSUER);
+
 	/* Disable the receiving and processing of FUSE_INTERRUPT requests */
 	conn->no_interrupt = 1;
 }

example/passthrough.c

@@ -53,7 +53,9 @@ static int readdir_zero_ino;
 static void *xmp_init(struct fuse_conn_info *conn,
 		      struct fuse_config *cfg)
 {
-	(void) conn;
+	/* Always replies inline on the io-uring worker thread */
+	fuse_set_conn_flag(conn, FUSE_CONN_FLAG_SINGLE_ISSUER);
+
 	cfg->use_ino = !readdir_zero_ino;
 
 	/* parallel_direct_writes feature depends on direct_io features.

example/passthrough_fh.c

@@ -52,7 +52,9 @@
 static void *xmp_init(struct fuse_conn_info *conn,
 		      struct fuse_config *cfg)
 {
-	(void) conn;
+	/* Always replies inline on the io-uring worker thread */
+	fuse_set_conn_flag(conn, FUSE_CONN_FLAG_SINGLE_ISSUER);
+
 	cfg->use_ino = 1;
 	cfg->nullpath_ok = 1;
 

example/passthrough_hp.cc

@@ -205,6 +205,9 @@ static void sfs_init(void *userdata, fuse_conn_info *conn)
 {
 	(void)userdata;
 
+	/* Always replies inline on the io-uring worker thread */
+	fuse_set_conn_flag(conn, FUSE_CONN_FLAG_SINGLE_ISSUER);
+
 	if (!fuse_set_feature_flag(conn, FUSE_CAP_PASSTHROUGH))
 		fs.passthrough = false;
 

example/passthrough_ll.c

@@ -167,6 +167,9 @@ static void lo_init(void *userdata,
 	struct lo_data *lo = (struct lo_data *)userdata;
 	bool has_flag;
 
+	/* Always replies inline on the io-uring worker thread */
+	fuse_set_conn_flag(conn, FUSE_CONN_FLAG_SINGLE_ISSUER);
+
 	if (lo->writeback) {
 		has_flag = fuse_set_feature_flag(conn, FUSE_CAP_WRITEBACK_CACHE);
 		if (lo->debug && has_flag)

example/service_hl.c

@@ -88,7 +88,9 @@ static struct service_hl hl = { };
 static void *service_hl_init(struct fuse_conn_info *conn,
 			struct fuse_config *cfg)
 {
-	(void) conn;
+	/* Always replies inline on the io-uring worker thread */
+	fuse_set_conn_flag(conn, FUSE_CONN_FLAG_SINGLE_ISSUER);
+
 	cfg->kernel_cache = 1;
 
 	return NULL;

example/service_ll.c

@@ -85,6 +85,9 @@ static void service_ll_init(void *userdata, struct fuse_conn_info *conn)
 {
 	(void)userdata;
 
+	/* Always replies inline on the io-uring worker thread */
+	fuse_set_conn_flag(conn, FUSE_CONN_FLAG_SINGLE_ISSUER);
+
 	conn->time_gran = 1;
 }