[Devel] [PATCH VZ10] fs/fuse kio: track pending kRPC connect via state machine only

[Konstantin Khorenko]

Liu, please explain what do i do with that patch?
Should i revert ("fs/fuse kio: fix kRPC connect issues") and push this patch instead?

Should we release it as an RK? If yes - for which kernels and where are the corresponding bugs?
Currently i understand nothing, sorry.

--
Best regards,

Konstantin Khorenko,
Virtuozzo Linux Kernel Team

On 8/27/26 14:37, Liu Kui wrote:
> Rework the previous fix ("fs/fuse kio: fix kRPC connect issues") to not
> require the new struct pcs_krpc member "connect_req", so the fix can be
> shipped as a livepatch.
> 
> Both things connect_req was tracking are already derivable from the
> existing state machine once PCS_KRPC_STATE_CONNECT is made to mean
> exactly "a connect req is in flight":
> 
>  - krpc_connect_done() settles a failed connect back to UNCONN instead
>    of leaving the state in CONNECT forever;
> 
>  - pcs_krpc_abort() no longer resets CONNECT to UNCONN: the req is
>    still in flight, and only its completion settles the state;
> 
>  - pcs_krpc_connect() proceeds only from UNCONN or ABORTED, refusing
>    new connects (-EPERM) while a req is in flight - at most one connect
>    req exists at a time, same as with the connect_req check;
> 
>  - pcs_krpc_poll() reports EPOLLERR on UNCONN: poll bails out earlier
>    unless ctx->gen == krpc->gen, and the current session can only be in
>    UNCONN if its connect failed or was aborted, which is what the
>    (CONNECT && !connect_req) test used to detect.
> 
> gen only advances in pcs_krpc_connect(), which is blocked during
> CONNECT, so within that state the in-flight req always carries the
> current gen and krpc_connect_done()'s existing staleness check is
> sufficient.
> 
> Related to:
> https://virtuozzo.atlassian.net/browse/VSTOR-135626
> 
> Signed-off-by: Liu Kui <kui.liu at virtuozzo.com>
> ---
>  fs/fuse/kio/pcs/pcs_krpc.c | 39 ++++++++++++++++++++++++++++----------
>  fs/fuse/kio/pcs/pcs_krpc.h |  2 --
>  2 files changed, 29 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/fuse/kio/pcs/pcs_krpc.c b/fs/fuse/kio/pcs/pcs_krpc.c
> index b55c093c13d0..bcfbfc6d9304 100644
> --- a/fs/fuse/kio/pcs/pcs_krpc.c
> +++ b/fs/fuse/kio/pcs/pcs_krpc.c
> @@ -738,8 +738,12 @@ static int pcs_krpc_abort(struct pcs_krpc *krpc)
>  	spin_lock(&krpc->lock);
>  
>  	if (krpc->state != PCS_KRPC_STATE_CONNECTED) {
> -		if (krpc->state == PCS_KRPC_STATE_CONNECT)
> -			krpc->state = PCS_KRPC_STATE_UNCONN;
> +		/*
> +		 * A pending connect stays in CONNECT state: its connect req
> +		 * is still in flight and krpc_connect_done() will settle the
> +		 * state to UNCONN when it completes.  Until then new connects
> +		 * are refused, so at most one connect req exists at a time.
> +		 */
>  		spin_unlock(&krpc->lock);
>  		return 0;
>  	}
> @@ -907,8 +911,13 @@ static __poll_t pcs_krpc_poll(struct file *file, poll_table *wait)
>  
>  	spin_lock(&krpc->lock);
>  
> +	/*
> +	 * ctx->gen == krpc->gen (checked above) means this is the current
> +	 * session, so UNCONN here can only mean its connect attempt has
> +	 * failed (see krpc_connect_done()) or the session was aborted.
> +	 */
>  	if (krpc->state == PCS_KRPC_STATE_ABORTED ||
> -	    (krpc->state == PCS_KRPC_STATE_CONNECT && !krpc->connect_req)) {
> +	    krpc->state == PCS_KRPC_STATE_UNCONN) {
>  		pollflags |= EPOLLERR;
>  	} else if (krpc->state == PCS_KRPC_STATE_CONNECTED) {
>  		pollflags |= EPOLLOUT;
> @@ -999,7 +1008,6 @@ int pcs_krpc_create(struct pcs_krpc_set *krpcs, PCS_NODE_ID_T *id,
>  	krpc->gen = 0;
>  	krpc->state = PCS_KRPC_STATE_UNCONN;
>  	krpc->cs = NULL;
> -	krpc->connect_req = NULL;
>  
>  	krpc->rpc = pcs_rpc_clnt_create(&cc_from_krpcset(krpcs)->eng, id, addr, cs_flags);
>  	if (!krpc->rpc) {
> @@ -1051,8 +1059,6 @@ static void krpc_connect_done(struct pcs_msg *msg)
>  	}
>  
>  	spin_lock(&krpc->lock);
> -	if (krpc->connect_req == req)
> -		krpc->connect_req = NULL;
>  	/* from a stale session, do nothing  */
>  	if (req->gen != krpc->gen || krpc->state != PCS_KRPC_STATE_CONNECT) {
>  		spin_unlock(&krpc->lock);
> @@ -1062,6 +1068,14 @@ static void krpc_connect_done(struct pcs_msg *msg)
>  	if (!pcs_if_error(&msg->error)) {
>  		krpc->state = PCS_KRPC_STATE_CONNECTED;
>  		pollflags = EPOLLOUT;
> +	} else {
> +		/*
> +		 * Connect failed: settle back to UNCONN so that a new connect
> +		 * is allowed again, and report the failure to poll().  Since
> +		 * gen is unchanged, the current session's poll sees UNCONN
> +		 * and returns EPOLLERR.
> +		 */
> +		krpc->state = PCS_KRPC_STATE_UNCONN;
>  	}
>  	spin_unlock(&krpc->lock);
>  
> @@ -1119,9 +1133,15 @@ int pcs_krpc_connect(struct pcs_krpc_set *krpcs, PCS_NODE_ID_T *id)
>  	}
>  
>  	spin_lock(&krpc->lock);
> -	if (krpc->state == PCS_KRPC_STATE_CONNECTED ||
> -	    krpc->state == PCS_KRPC_STATE_DESTROYED ||
> -	    krpc->connect_req) {
> +	/*
> +	 * A connect is allowed only when there is neither an established
> +	 * session nor a connect req in flight (CONNECT state, see
> +	 * krpc_connect_done()).  This limits connect reqs to one at a time:
> +	 * if userspace gave up on a connect and retries, the new connect
> +	 * fails immediately until the old req completes.
> +	 */
> +	if (krpc->state != PCS_KRPC_STATE_UNCONN &&
> +	    krpc->state != PCS_KRPC_STATE_ABORTED) {
>  		spin_unlock(&krpc->lock);
>  		err = -EPERM;
>  		/* fput() drops ctx and its krpc reference via pcs_krpc_release() */
> @@ -1133,7 +1153,6 @@ int pcs_krpc_connect(struct pcs_krpc_set *krpcs, PCS_NODE_ID_T *id)
>  	connect_req->gen = krpc->gen;
>  	connect_req->krpc = pcs_krpc_get(krpc);
>  	krpc->state = PCS_KRPC_STATE_CONNECT;
> -	krpc->connect_req = connect_req;
>  	spin_unlock(&krpc->lock);
>  
>  	/* publish the fd only after the connect is committed */
> diff --git a/fs/fuse/kio/pcs/pcs_krpc.h b/fs/fuse/kio/pcs/pcs_krpc.h
> index 3db9e9712019..6a090ef66185 100644
> --- a/fs/fuse/kio/pcs/pcs_krpc.h
> +++ b/fs/fuse/kio/pcs/pcs_krpc.h
> @@ -82,8 +82,6 @@ struct pcs_krpc {
>  	/** Wait queue head for poll */
>  	wait_queue_head_t		poll_wait;
>  	struct pcs_cs			*cs;
> -
> -	struct krpc_connect_req *connect_req;
>  };
>  
>  struct pcs_krpc_context {