Re: [PATCH v2] thunderbolt: Fix tb->lock deadlock during hot-unplug on AMD USB4 routers

[Mario Limonciello <mario.limonciello@amd.com>]

On 8/25/26 16:42, juan.martinez@amd.com wrote:
> From: Juan Martinez <juan.martinez@amd.com>
> 
> Commit f1de1fc5f632 ("thunderbolt: Add quirk to reset host interface on
> DMA path teardown for AMD USB4 routers") introduced a deadlock when
> physically unplugging a Thunderbolt cable on AMD systems.
> 
> The problem occurs because tb_handle_hotplug() holds tb->lock while
> processing the unplug event. When it removes the XDomain services,
> tbnet_remove() calls tb_xdomain_disable_paths() which eventually calls
> tb_domain_reset_interface(). That function tries to acquire tb->lock
> via guard(mutex), but the hotplug worker already holds it, causing a
> self-deadlock.
> 
> The deadlock manifests as a complete network hang because
> tb_handle_hotplug() holds RTNL while waiting on its own mutex, blocking
> all network operations system-wide.
> 
> The existing code already handles this scenario partially: when
> xd->is_unplugged is true, tb_disconnect_xdomain_paths() intentionally
> skips the DMA teardown because the hotplug handler will do it later
> via __tb_disconnect_xdomain_paths(). However, the reset was still
> being called unconditionally.
> 
> Fix this by:
> 1. Splitting tb_domain_reset_interface() into a locked inner function
>     __tb_domain_reset_interface_locked() and a locking wrapper
> 2. Skipping the reset in tb_domain_disconnect_xdomain_paths() when
>     xd->is_unplugged is true (matching the existing teardown skip logic)
> 3. Calling __tb_domain_reset_interface_locked() from tb_handle_hotplug()
>     after __tb_disconnect_xdomain_paths() where the actual DMA teardown
>     happens and tb->lock is already held
> 
> This preserves the reset behavior for normal shutdown paths while
> avoiding the deadlock during physical cable unplug.
> 
> Fixes: f1de1fc5f632 ("thunderbolt: Add quirk to reset host interface on DMA path teardown for AMD USB4 routers")
> Signed-off-by: Juan Martinez <juan.martinez@amd.com>
> ---
>   drivers/thunderbolt/domain.c | 26 +++++++++++++++++++++-----
>   drivers/thunderbolt/tb.c     |  1 +
>   drivers/thunderbolt/tb.h     |  1 +
>   3 files changed, 23 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/thunderbolt/domain.c b/drivers/thunderbolt/domain.c
> index 12c88509a54f..253ea8c6b757 100644
> --- a/drivers/thunderbolt/domain.c
> +++ b/drivers/thunderbolt/domain.c
> @@ -788,14 +788,19 @@ int tb_domain_approve_xdomain_paths(struct tb *tb, struct tb_xdomain *xd,
>   			transmit_ring, receive_path, receive_ring);
>   }
>   
> -static void tb_domain_reset_interface(struct tb *tb)
> +/*
> + * __tb_domain_reset_interface_locked - Reset host interface (lock held)
> + *
> + * Caller must hold tb->lock. Used by hotplug path where lock is already held.
> + */
> +void __tb_domain_reset_interface_locked(struct tb *tb)
>   {
>   	struct tb_nhi *nhi = tb->nhi;
>   
> -	if (!nhi->ops->reset_interface)
> -		return;

I'm not sure this is correct to move from here to 
tb_domain_reset_interface() because you still call 
__tb_domain_reset_interface_locked() from tb_handle_hotplug() which 
doesn't do this check.

> +	lockdep_assert_held(&tb->lock);
>   
> -	guard(mutex)(&tb->lock);
> +	if (!(nhi->quirks & QUIRK_RESET_DMA_ON_TEARDOWN))
> +		return;
>   
>   	/* The reset clears the ring state so stop the control channel */
>   	tb_ctl_stop(tb->ctl);
> @@ -803,6 +808,17 @@ static void tb_domain_reset_interface(struct tb *tb)
>   	tb_ctl_start(tb->ctl);
>   }
>   
> +static void tb_domain_reset_interface(struct tb *tb)
> +{
> +	struct tb_nhi *nhi = tb->nhi;
> +
> +	if (!nhi->ops->reset_interface)
> +		return;
> +
> +	guard(mutex)(&tb->lock);
> +	__tb_domain_reset_interface_locked(tb);
> +}
> +
>   /**
>    * tb_domain_disconnect_xdomain_paths() - Disable DMA paths for XDomain
>    * @tb: Domain disabling the DMA paths
> @@ -835,7 +851,7 @@ int tb_domain_disconnect_xdomain_paths(struct tb *tb, struct tb_xdomain *xd,
>   	if (ret)
>   		return ret;
>   
> -	if (tb->nhi->quirks & QUIRK_RESET_DMA_ON_TEARDOWN)
> +	if (!xd->is_unplugged)
>   		tb_domain_reset_interface(tb);
>   
>   	return 0;
> diff --git a/drivers/thunderbolt/tb.c b/drivers/thunderbolt/tb.c
> index b7cc6894a598..89dfb3381345 100644
> --- a/drivers/thunderbolt/tb.c
> +++ b/drivers/thunderbolt/tb.c
> @@ -2489,6 +2489,7 @@ static void tb_handle_hotplug(struct work_struct *work)
>   			tb_xdomain_remove(xd);
>   			port->xdomain = NULL;
>   			__tb_disconnect_xdomain_paths(tb, xd, -1, -1, -1, -1);
> +			__tb_domain_reset_interface_locked(tb);
>   			tb_xdomain_put(xd);
>   			tb_port_unconfigure_xdomain(port);
>   		} else if (tb_port_is_dpout(port) || tb_port_is_dpin(port)) {
> diff --git a/drivers/thunderbolt/tb.h b/drivers/thunderbolt/tb.h
> index 4373336d9425..2e6e0920cb1f 100644
> --- a/drivers/thunderbolt/tb.h
> +++ b/drivers/thunderbolt/tb.h
> @@ -789,6 +789,7 @@ int tb_domain_disconnect_pcie_paths(struct tb *tb);
>   int tb_domain_approve_xdomain_paths(struct tb *tb, struct tb_xdomain *xd,
>   				    int transmit_path, int transmit_ring,
>   				    int receive_path, int receive_ring);
> +void __tb_domain_reset_interface_locked(struct tb *tb);
>   int tb_domain_disconnect_xdomain_paths(struct tb *tb, struct tb_xdomain *xd,
>   				       int transmit_path, int transmit_ring,
>   				       int receive_path, int receive_ring);