[PATCH RFC v7 15/16] fuse: {io-uring} Prevent mount point hang on fuse-server termination

Bernd Schubert
2024-11-27T13:41:03Z

When the fuse-server terminates while the fuse-client or kernel
still has queued URING_CMDs, these commands retain references
to the struct file used by the fuse connection. This prevents
fuse_dev_release() from being invoked, resulting in a hung mount
point.

This patch addresses the issue by making queued URING_CMDs
cancelable, allowing fuse_dev_release() to proceed as expected
and preventing the mount point from hanging.

Signed-off-by: Bernd Schubert <bschubert@ddn.com>
---
fs/fuse/dev_uring.c | 109 ++++++++++++++++++++++++++++++++++++++------------
fs/fuse/dev_uring_i.h | 12 ++++++
2 files changed, 95 insertions(+), 26 deletions(-)

diff --git a/fs/fuse/dev_uring.c b/fs/fuse/dev_uring.c
index 94dc3f56d4ab604eb4b87d3b9731567e3a214b0a..fe24e31bbfecec526f88bc5b82b0aa132357c1cc 100644
--- a/fs/fuse/dev_uring.c
+++ b/fs/fuse/dev_uring.c
@@ -26,6 +26,7 @@ bool fuse_uring_enabled(void)

struct fuse_uring_cmd_pdu {
struct fuse_ring_ent *ring_ent;
+ struct fuse_ring_queue *queue;
};

const struct fuse_iqueue_ops fuse_io_uring_ops;
@@ -221,6 +222,7 @@ static struct fuse_ring_queue *fuse_uring_create_queue(struct fuse_ring *ring,
struct fuse_conn *fc = ring->fc;
struct fuse_ring_queue *queue;
struct list_head *pq;
+ struct fuse_ring_ent *ent, *next;

queue = kzalloc(sizeof(*queue), GFP_KERNEL_ACCOUNT);
if (!queue)
@@ -249,6 +251,12 @@ static struct fuse_ring_queue *fuse_uring_create_queue(struct fuse_ring *ring,
INIT_LIST_HEAD(&queue->ent_in_userspace);
INIT_LIST_HEAD(&queue->fuse_req_queue);
INIT_LIST_HEAD(&queue->fuse_req_bg_queue);
+ INIT_LIST_HEAD(&queue->ent_released);
+
+ list_for_each_entry_safe(ent, next, &queue->ent_released, list) {
+ list_del_init(&ent->list);
+ kfree(ent);
+ }

queue->fpq.processing = pq;
fuse_pqueue_init(&queue->fpq);
@@ -281,24 +289,27 @@ static void fuse_uring_stop_fuse_req_end(struct fuse_ring_ent *ent)
/*
* Release a request/entry on connection tear down
*/
-static void fuse_uring_entry_teardown(struct fuse_ring_ent *ent,
- bool need_cmd_done)
+static void fuse_uring_entry_teardown(struct fuse_ring_ent *ent)
{
- /*
- * fuse_request_end() might take other locks like fi->lock and
- * can lead to lock ordering issues
- */
- lockdep_assert_not_held(&ent->queue->lock);
+ struct fuse_ring_queue *queue = ent->queue;

- if (need_cmd_done)
+ if (ent->need_cmd_done)
io_uring_cmd_done(ent->cmd, -ENOTCONN, 0,
IO_URING_F_UNLOCKED);

if (ent->fuse_req)
fuse_uring_stop_fuse_req_end(ent);

- list_del_init(&ent->list);
- kfree(ent);
+ /*
+ * The entry must not be freed immediately, due to access of direct
+ * pointer access of entries through IO_URING_F_CANCEL - there is a risk
+ * of race between daemon termination (which triggers IO_URING_F_CANCEL
+ * and accesses entries without checking the list state first
+ */
+ spin_lock(&queue->lock);
+ list_move(&ent->list, &queue->ent_released);
+ ent->state = FRRS_RELEASED;
+ spin_unlock(&queue->lock);
}

static void fuse_uring_stop_list_entries(struct list_head *head,
@@ -318,15 +329,15 @@ static void fuse_uring_stop_list_entries(struct list_head *head,
continue;
}

+ ent->need_cmd_done = ent->state != FRRS_USERSPACE;
+ ent->state = FRRS_TEARDOWN;
list_move(&ent->list, &to_teardown);
}
spin_unlock(&queue->lock);

/* no queue lock to avoid lock order issues */
list_for_each_entry_safe(ent, next, &to_teardown, list) {
- bool need_cmd_done = ent->state != FRRS_USERSPACE;
-
- fuse_uring_entry_teardown(ent, need_cmd_done);
+ fuse_uring_entry_teardown(ent);
queue_refs = atomic_dec_return(&ring->queue_refs);

WARN_ON_ONCE(queue_refs < 0);
@@ -434,6 +445,49 @@ void fuse_uring_stop_queues(struct fuse_ring *ring)
}
}

+/*
+ * Handle IO_URING_F_CANCEL, typically should come on daemon termination
+ */
+static void fuse_uring_cancel(struct io_uring_cmd *cmd,
+ unsigned int issue_flags, struct fuse_conn *fc)
+{
+ struct fuse_uring_cmd_pdu *pdu = (struct fuse_uring_cmd_pdu *)cmd->pdu;
+ struct fuse_ring_queue *queue = pdu->queue;
+ struct fuse_ring_ent *ent = pdu->ring_ent;
+ bool need_cmd_done = false;
+
+ /*
+ * direct access on ent - it must not be destructed as long as
+ * IO_URING_F_CANCEL might come up
+ */
+ spin_lock(&queue->lock);
+ if (ent->state == FRRS_WAIT) {
+ ent->state = FRRS_USERSPACE;
+ list_move(&ent->list, &queue->ent_in_userspace);
+ need_cmd_done = true;
+ }
+ spin_unlock(&queue->lock);
+
+ if (need_cmd_done)
+ io_uring_cmd_done(cmd, -ENOTCONN, 0, issue_flags);
+
+ /*
+ * releasing the last entry should trigger fuse_dev_release() if
+ * the daemon was terminated
+ */
+}
+
+static void fuse_uring_prepare_cancel(struct io_uring_cmd *cmd, int issue_flags,
+ struct fuse_ring_ent *ring_ent)
+{
+ struct fuse_uring_cmd_pdu *pdu = (struct fuse_uring_cmd_pdu *)cmd->pdu;
+
+ pdu->ring_ent = ring_ent;
+ pdu->queue = ring_ent->queue;
+
+ io_uring_cmd_mark_cancelable(cmd, issue_flags);
+}
+
/*
* Checks for errors and stores it into the request
*/
@@ -638,8 +692,10 @@ static int fuse_uring_send_next_to_ring(struct fuse_ring_ent *ring_ent)
* Make a ring entry available for fuse_req assignment
*/
static void fuse_uring_ent_avail(struct fuse_ring_ent *ring_ent,
- struct fuse_ring_queue *queue)
+ struct fuse_ring_queue *queue,
+ unsigned int issue_flags)
{
+ fuse_uring_prepare_cancel(ring_ent->cmd, issue_flags, ring_ent);
list_move(&ring_ent->list, &queue->ent_avail_queue);
ring_ent->state = FRRS_WAIT;
}
@@ -742,7 +798,8 @@ static void fuse_uring_commit(struct fuse_ring_ent *ring_ent,
* Get the next fuse req and send it
*/
static void fuse_uring_next_fuse_req(struct fuse_ring_ent *ring_ent,
- struct fuse_ring_queue *queue)
+ struct fuse_ring_queue *queue,
+ unsigned int issue_flags)
{
int has_next, err;
int prev_state = ring_ent->state;
@@ -751,7 +808,7 @@ static void fuse_uring_next_fuse_req(struct fuse_ring_ent *ring_ent,
spin_lock(&queue->lock);
has_next = fuse_uring_ent_assign_req(ring_ent);
if (!has_next) {
- fuse_uring_ent_avail(ring_ent, queue);
+ fuse_uring_ent_avail(ring_ent, queue, issue_flags);
spin_unlock(&queue->lock);
break; /* no request left */
}
@@ -826,7 +883,7 @@ static int fuse_uring_commit_fetch(struct io_uring_cmd *cmd, int issue_flags,
* and fetching is done in one step vs legacy fuse, which has separated
* read (fetch request) and write (commit result).
*/
- fuse_uring_next_fuse_req(ring_ent, queue);
+ fuse_uring_next_fuse_req(ring_ent, queue, issue_flags);
return 0;
}

@@ -868,7 +925,7 @@ static void _fuse_uring_fetch(struct fuse_ring_ent *ring_ent,
struct fuse_iqueue *fiq = &fc->iq;

spin_lock(&queue->lock);
- fuse_uring_ent_avail(ring_ent, queue);
+ fuse_uring_ent_avail(ring_ent, queue, issue_flags);
ring_ent->cmd = cmd;
spin_unlock(&queue->lock);

@@ -1022,6 +1079,11 @@ int fuse_uring_cmd(struct io_uring_cmd *cmd, unsigned int issue_flags)
if (!fc->connected || fc->aborted)
return fc->aborted ? -ECONNABORTED : -ENOTCONN;

+ if ((unlikely(issue_flags & IO_URING_F_CANCEL))) {
+ fuse_uring_cancel(cmd, issue_flags, fc);
+ return 0;
+ }
+
switch (cmd_op) {
case FUSE_URING_REQ_FETCH:
err = fuse_uring_fetch(cmd, issue_flags, fc);
@@ -1074,7 +1136,7 @@ fuse_uring_send_req_in_task(struct io_uring_cmd *cmd,

return;
err:
- fuse_uring_next_fuse_req(ring_ent, queue);
+ fuse_uring_next_fuse_req(ring_ent, queue, issue_flags);
}

static struct fuse_ring_queue *fuse_uring_task_to_queue(struct fuse_ring *ring)
@@ -1129,14 +1191,11 @@ void fuse_uring_queue_fuse_req(struct fuse_iqueue *fiq, struct fuse_req *req)

if (ring_ent) {
struct io_uring_cmd *cmd = ring_ent->cmd;
- struct fuse_uring_cmd_pdu *pdu =
- (struct fuse_uring_cmd_pdu *)cmd->pdu;
-
err = -EIO;
if (WARN_ON_ONCE(ring_ent->state != FRRS_FUSE_REQ))
goto err;

- pdu->ring_ent = ring_ent;
+ /* pdu already set by preparing IO_URING_F_CANCEL */
io_uring_cmd_complete_in_task(cmd, fuse_uring_send_req_in_task);
}

@@ -1189,12 +1248,10 @@ bool fuse_uring_queue_bq_req(struct fuse_req *req)
list);
if (ring_ent && req) {
struct io_uring_cmd *cmd = ring_ent->cmd;
- struct fuse_uring_cmd_pdu *pdu =
- (struct fuse_uring_cmd_pdu *)cmd->pdu;

fuse_uring_add_req_to_ring_ent(ring_ent, req);

- pdu->ring_ent = ring_ent;
+ /* pdu already set by preparing IO_URING_F_CANCEL */
io_uring_cmd_complete_in_task(cmd, fuse_uring_send_req_in_task);
}
spin_unlock(&queue->lock);
diff --git a/fs/fuse/dev_uring_i.h b/fs/fuse/dev_uring_i.h
index 0826fb1c03e2e38dedad56552ea09461965e248f..df541247f07e413923b13b6bf203f301e8c1710a 100644
--- a/fs/fuse/dev_uring_i.h
+++ b/fs/fuse/dev_uring_i.h
@@ -28,6 +28,12 @@ enum fuse_ring_req_state {

/* The ring entry is in or on the way to user space */
FRRS_USERSPACE,
+
+ /* The ring entry is in teardown */
+ FRRS_TEARDOWN,
+
+ /* The ring entry is released, but not freed yet */
+ FRRS_RELEASED,
};

/** A fuse ring entry, part of the ring queue */
@@ -52,6 +58,9 @@ struct fuse_ring_ent {
*/
unsigned int state;

+ /* The entry needs io_uring_cmd_done for teardown */
+ unsigned int need_cmd_done;
+
struct fuse_req *fuse_req;
};

@@ -84,6 +93,9 @@ struct fuse_ring_queue {
/* entries in userspace */
struct list_head ent_in_userspace;

+ /* entries that are released */
+ struct list_head ent_released;
+
/* fuse requests waiting for an entry slot */
struct list_head fuse_req_queue;

--
2.43.0