[PATCH v3 3/3] fuse: publish io-uring queues with release semantics

fuse_uring_create_queue() initializes a fuse_ring_queue and then
publishes the pointer into ring->queues[qid] with WRITE_ONCE() under the
fch->lock. There are several readers that may concurrently be fetching
that pointer locklessly and then deferencing it.

WRITE_ONCE() doesn't ensure ordering of the queue's field
initialization before the ring->queues[qid] pointer assignment. The
queue must be published with smp_store_release() so the field
initialization is guaranteed to happen before.

Readers in paths where the read may happen concurrently with the store
need to use READ_ONCE() because any race involving a plain access is
undefined.

Fixes: 24fe962c86f5 ("fuse: {io-uring} Handle SQEs - register commands")
Cc: [email protected]
Reviewed-by: Bernd Schubert <[email protected]>
Signed-off-by: Joanne Koong <[email protected]>
---
 fs/fuse/dev_uring.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/fs/fuse/dev_uring.c b/fs/fuse/dev_uring.c
index 51f985154aa1..c8488ebc1d1f 100644
--- a/fs/fuse/dev_uring.c
+++ b/fs/fuse/dev_uring.c
@@ -204,7 +204,7 @@ void fuse_uring_destruct(struct fuse_chan *fch)
 		return;
 
 	for (qid = 0; qid < ring->nr_queues; qid++) {
-		struct fuse_ring_queue *queue = ring->queues[qid];
+		struct fuse_ring_queue *queue = READ_ONCE(ring->queues[qid]);
 		struct fuse_ring_ent *ent, *next;
 
 		if (!queue)
@@ -223,7 +223,7 @@ void fuse_uring_destruct(struct fuse_chan *fch)
 
 		kfree(queue->fpq.processing);
 		kfree(queue);
-		ring->queues[qid] = NULL;
+		WRITE_ONCE(ring->queues[qid], NULL);
 	}
 
 	kfree(ring->queues);
@@ -321,9 +321,11 @@ static struct fuse_ring_queue *fuse_uring_create_queue(struct fuse_ring *ring,
 	}
 
 	/*
-	 * write_once and lock as the caller mostly doesn't take the lock at all
+	 * fch->lock serializes concurrent creators for this qid.
+	 * smp_store_release() are for the lockless readers who must see a
+	 * fully initialized queue after &ring->queues[qid] is set
 	 */
-	WRITE_ONCE(ring->queues[qid], queue);
+	smp_store_release(&ring->queues[qid], queue);
 	spin_unlock(&fch->lock);
 
 	return queue;
@@ -434,7 +436,7 @@ static void fuse_uring_log_ent_state(struct fuse_ring *ring)
 	struct fuse_ring_ent *ent;
 
 	for (qid = 0; qid < ring->nr_queues; qid++) {
-		struct fuse_ring_queue *queue = ring->queues[qid];
+		struct fuse_ring_queue *queue = READ_ONCE(ring->queues[qid]);
 
 		if (!queue)
 			continue;
@@ -967,7 +969,7 @@ static int fuse_uring_commit_fetch(struct io_uring_cmd *cmd, int issue_flags,
 	if (qid >= ring->nr_queues)
 		return -EINVAL;
 
-	queue = ring->queues[qid];
+	queue = READ_ONCE(ring->queues[qid]);
 	if (!queue)
 		return err;
 	fpq = &queue->fpq;
@@ -1035,7 +1037,7 @@ static bool is_ring_ready(struct fuse_ring *ring, int current_qid)
 		if (current_qid == qid)
 			continue;
 
-		queue = ring->queues[qid];
+		queue = READ_ONCE(ring->queues[qid]);
 		if (!queue) {
 			ready = false;
 			break;
@@ -1191,7 +1193,7 @@ static int fuse_uring_register(struct io_uring_cmd *cmd,
 		return -EINVAL;
 	}
 
-	queue = ring->queues[qid];
+	queue = READ_ONCE(ring->queues[qid]);
 	if (!queue) {
 		queue = fuse_uring_create_queue(ring, qid);
 		if (!queue)
@@ -1332,7 +1334,7 @@ static struct fuse_ring_queue *fuse_uring_task_to_queue(struct fuse_ring *ring)
 		      ring->nr_queues))
 		qid = 0;
 
-	queue = ring->queues[qid];
+	queue = READ_ONCE(ring->queues[qid]);
 	WARN_ONCE(!queue, "Missing queue for qid %d\n", qid);
 
 	return queue;
-- 
2.52.0