fuse_uring: restore inline-reply batching, add single-issuer fast path

Part 1 dropped cqe_processing and made the serving loop wait-only, so
the default path now issues one io_uring_submit() per inline reply
instead of coalescing a cqe batch into a single submit.

Restore the batching: the loop splits submit_and_wait so the flush
stays under ring_lock (part 1's race fix), and cqe_processing - now an
_Atomic bool set lock-free around handle_cqes() - again gates the
per-reply submit. The SQ stays serialised, so a batched reply is always
flushed before the next wait, never stranded.

Add a conn flag, io_uring_single_issuer, for a filesystem that replies
only from the per-queue uring thread: it skips ring_lock entirely and
runs the combined io_uring_submit_and_wait() loop, sound because only
that thread touches the ring. The flag reuses a reserved padding bit
(ABI unchanged; flag 0 keeps the multi-issuer path) and is mirrored
into the ring pool after op.init(), before serving starts.

A filesystem enables it with fuse_set_conn_flag(conn,
FUSE_CONN_FLAG_SINGLE_ISSUER), a new exported setter modelled on
fuse_set_feature_flag(); the io_uring_single_issuer bitfield is the
internal storage.

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

include/fuse_common.h

@@ -698,8 +698,19 @@ struct fuse_conn_info {
 	 */
 	uint32_t no_interrupt : 1;
 
+	/**
+	 * Only meaningful with io-uring (FUSE_CAP_OVER_IO_URING).
+
+	 * The filesystem promises that every reply to an io-uring request is
+	 * sent from the same thread that received the request (the per-queue
+	 * io-uring worker), i.e. replies are never deferred to another thread.
+	 * The flag is used for io-uring optimizations.
+	 *
+	 */
+	uint32_t io_uring_single_issuer : 1;
+
 	/* reserved bits for future use */
-	uint32_t padding : 31;
+	uint32_t padding : 30;
 
 	/**
 	 * Extended capability flags that the kernel supports (read-only)
@@ -1140,6 +1151,22 @@ void fuse_unset_feature_flag(struct fuse_conn_info *conn, uint64_t flag);
  */
 bool fuse_get_feature_flag(const struct fuse_conn_info *conn, uint64_t flag);
 
+/* The file system replies to requests from the same thread that received them,
+ * allowing io-uring optimizations
+ */
+#define FUSE_CONN_FLAG_SINGLE_ISSUER (1u << 0)
+
+/**
+ * Set a libfuse connection flag (a FUSE_CONN_FLAG_* value). Unlike
+ * fuse_set_feature_flag(), these are libfuse-side hints, not negotiated
+ * with the kernel.
+ *
+ * @param conn connection information
+ * @param flag a single FUSE_CONN_FLAG_* value
+ * @return true if the flag is known to this libfuse and was set, false otherwise
+ */
+bool fuse_set_conn_flag(struct fuse_conn_info *conn, uint64_t flag);
+
 /*
  * DO NOT USE: Not part of public API, for internal test use only.
  * The function signature or any use of it is not guaranteeed to

lib/fuse_lowlevel.c

@@ -2700,6 +2700,17 @@ bool fuse_get_feature_flag(const struct fuse_conn_info *conn,
 	return (conn->capable_ext & flag) ? true : false;
 }
 
+bool fuse_set_conn_flag(struct fuse_conn_info *conn, uint64_t flag)
+{
+	switch (flag) {
+	case FUSE_CONN_FLAG_SINGLE_ISSUER:
+		conn->io_uring_single_issuer = 1;
+		return true;
+	default:
+		return false;
+	}
+}
+
 /* Prevent bogus data races (bogus since "init" is called before
  * multi-threading becomes relevant */
 static __attribute__((no_sanitize("thread"))) void

lib/fuse_uring.c

@@ -75,6 +75,9 @@ struct fuse_ring_queue {
 struct fuse_ring_pool {
 	struct fuse_session *se;
 
+	/* mirror of se->conn.io_uring_single_issuer, fixed at ring creation */
+	bool single_issuer;
+
 	/* number of queues */
 	size_t nr_queues;
 
@@ -162,6 +165,7 @@ static int fuse_uring_commit_sqe(struct fuse_ring_pool *ring_pool,
 				 struct fuse_ring_queue *queue,
 				 struct fuse_ring_ent *ring_ent)
 {
+	const bool locked = !ring_pool->single_issuer;
 	struct fuse_session *se = ring_pool->se;
 	struct fuse_uring_req_header *rrh = ring_ent->req_header;
 	struct fuse_out_header *out = (struct fuse_out_header *)&rrh->in_out;
@@ -169,7 +173,14 @@ static int fuse_uring_commit_sqe(struct fuse_ring_pool *ring_pool,
 		(struct fuse_uring_ent_in_out *)&rrh->ring_ent_in_out;
 	struct io_uring_sqe *sqe;
 
-	pthread_mutex_lock(&queue->ring_lock);
+	/*
+	 * Multi-issuer: serialise every submission-side SQ access under
+	 * ring_lock. Single-issuer: only the uring thread submits, so skip the
+	 * lock and batch inline replies (cqe_processing), flushed by the next
+	 * submit_and_wait().
+	 */
+	if (locked)
+		pthread_mutex_lock(&queue->ring_lock);
 
 	sqe = io_uring_get_sqe(&queue->ring);
 
@@ -180,7 +191,8 @@ static int fuse_uring_commit_sqe(struct fuse_ring_pool *ring_pool,
 		 * SQEs matches the number tof requests.
 		 */
 
-		pthread_mutex_unlock(&queue->ring_lock);
+		if (locked)
+			pthread_mutex_unlock(&queue->ring_lock);
 		se->error = -EIO;
 		fuse_log(FUSE_LOG_ERR, "Failed to get a ring SQEs\n");
 
@@ -197,14 +209,11 @@ static int fuse_uring_commit_sqe(struct fuse_ring_pool *ring_pool,
 			 out->unique, ent_in_out->payload_sz);
 	}
 
-	/*
-	 * only submit when the main thread processing is not running. I.e.
-	 * No submission per request, but aggregated submission, if possible
-	 */
-	if (!queue->cqe_processing)
+	if (!atomic_load_explicit(&queue->cqe_processing, memory_order_relaxed))
 		io_uring_submit(&queue->ring);
 
-	pthread_mutex_unlock(&queue->ring_lock);
+	if (locked)
+		pthread_mutex_unlock(&queue->ring_lock);
 
 	return 0;
 }
@@ -542,6 +551,7 @@ static struct fuse_ring_pool *fuse_create_ring(struct fuse_session *se)
 	fuse_ring->queue_depth = se->uring.q_depth;
 	fuse_ring->max_req_payload_sz = payload_sz;
 	fuse_ring->queue_mem_size = queue_sz;
+	fuse_ring->single_issuer = se->conn.io_uring_single_issuer;
 
 	/*
 	 * very basic queue initialization, that cannot fail and will
@@ -575,9 +585,11 @@ err:
 static void fuse_uring_resubmit(struct fuse_ring_queue *queue,
 				struct fuse_ring_ent *ent)
 {
+	const bool locked = !queue->ring_pool->single_issuer;
 	struct io_uring_sqe *sqe;
 
-	pthread_mutex_lock(&queue->ring_lock);
+	if (locked)
+		pthread_mutex_lock(&queue->ring_lock);
 
 	sqe = io_uring_get_sqe(&queue->ring);
 	if (sqe == NULL) {
@@ -587,7 +599,8 @@ static void fuse_uring_resubmit(struct fuse_ring_queue *queue,
 		 * SQEs matches the number tof requests.
 		 */
 
-		pthread_mutex_unlock(&queue->ring_lock);
+		if (locked)
+			pthread_mutex_unlock(&queue->ring_lock);
 		queue->ring_pool->se->error = -EIO;
 		fuse_log(FUSE_LOG_ERR, "Failed to get a ring SQEs\n");
 
@@ -614,8 +627,10 @@ static void fuse_uring_resubmit(struct fuse_ring_queue *queue,
 		break;
 	}
 
-	io_uring_submit(&queue->ring);
-	pthread_mutex_unlock(&queue->ring_lock);
+	if (!atomic_load_explicit(&queue->cqe_processing, memory_order_relaxed))
+		io_uring_submit(&queue->ring);
+	if (locked)
+		pthread_mutex_unlock(&queue->ring_lock);
 }
 
 static void fuse_uring_handle_cqe(struct fuse_ring_queue *queue,
@@ -833,6 +848,7 @@ static void *fuse_uring_thread(void *arg)
 	struct fuse_ring_queue *queue = arg;
 	struct fuse_ring_pool *ring_pool = queue->ring_pool;
 	struct fuse_session *se = ring_pool->se;
+	const bool single_issuer = ring_pool->single_issuer;
 	int err;
 	char thread_name[16] = { 0 };
 
@@ -859,34 +875,57 @@ static void *fuse_uring_thread(void *arg)
 	sem_wait(&ring_pool->init_sem);
 
 	/*
-	 * Submit the registration SQEs once. Safe without ring_lock: no
-	 * request can reach this queue until the kernel sees these, so there
-	 * is no concurrent submitter yet.
+	 * Multi-issuer flushes the registration SQEs here - safe without
+	 * ring_lock, no request can reach this queue yet. Single-issuer's
+	 * first submit_and_wait() below flushes them instead.
 	 */
-	io_uring_submit(&queue->ring);
+	if (!single_issuer)
+		io_uring_submit(&queue->ring);
 
 	/* Not using fuse_session_exited(se), as that cannot be inlined */
 	while (!atomic_load_explicit(&se->mt_exited, memory_order_relaxed)) {
-		struct io_uring_cqe *cqe;
+		/*
+		 * Single-issuer: one combined submit_and_wait() flushes the
+		 * previous iteration's batched replies and waits. Multi-issuer:
+		 * split it - wait only here (no ring_lock) so off-thread
+		 * repliers keep submitting; the batched inline replies are
+		 * flushed below. The returned cqe is ignored; handle_cqes()
+		 * re-scans and advances the CQ.
+		 */
+		if (single_issuer) {
+			io_uring_submit_and_wait(&queue->ring, 1);
+		} else {
+			struct io_uring_cqe *cqe;
+
+			io_uring_wait_cqe(&queue->ring, &cqe);
+		}
 
 		/*
-		 * Wait only, not submit_and_wait: the serving loop must not
-		 * flush the SQ unlocked. All SQ flushing happens under ring_lock
-		 * - in commit_sqe()/resubmit() and the batched submit below. The
-		 * returned cqe is ignored; handle_cqes() re-scans the CQ.
+		 * Batch inline replies (commit_sqe()/resubmit()) across cqe
+		 * handling. Lock-free: the flag only gates who submits, while
+		 * the SQ stays serialised by ring_lock, so a reply that batched
+		 * here is always flushed by the submit below before the next
+		 * wait - it is never stranded.
 		 */
-		io_uring_wait_cqe(&queue->ring, &cqe);
-
-		queue->cqe_processing = true;
+		atomic_store_explicit(&queue->cqe_processing, true,
+				      memory_order_relaxed);
 
 		err = fuse_uring_queue_handle_cqes(queue);
 		if (err < 0)
 			goto err;
 
-		pthread_mutex_lock(&queue->ring_lock);
-		queue->cqe_processing = false;
-		io_uring_submit(&queue->ring);
-		pthread_mutex_unlock(&queue->ring_lock);
+		atomic_store_explicit(&queue->cqe_processing, false,
+				      memory_order_relaxed);
+
+		/*
+		 * Multi-issuer does not use io_uring_submit_and_wait(),
+		 * but io_uring_wait_cqe() and locked io_uring_submit().
+		 */
+		if (!single_issuer) {
+			pthread_mutex_lock(&queue->ring_lock);
+			io_uring_submit(&queue->ring);
+			pthread_mutex_unlock(&queue->ring_lock);
+		}
 	}
 
 	return NULL;

lib/fuse_versionscript

@@ -224,6 +224,7 @@ FUSE_3.18 {
 
 FUSE_3.19 {
 	global:
+		fuse_set_conn_flag;
 		fuse_session_start_teardown_watchdog;
 		fuse_session_stop_teardown_watchdog;
 		fuse_lowlevel_notify_prune;