[PATCH v4] thunderbolt: Fix tb->lock deadlock during hot-unplug on AMD USB4 routers

[Juan Martinez]

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@xxxxxxx>
---
 drivers/thunderbolt/domain.c | 20 +++++++++++++++++---
 drivers/thunderbolt/tb.c     |  1 +
 drivers/thunderbolt/tb.h     |  1 +
 3 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/drivers/thunderbolt/domain.c b/drivers/thunderbolt/domain.c
index 12c88509a54f..4cef9f4de523 100644
--- a/drivers/thunderbolt/domain.c
+++ b/drivers/thunderbolt/domain.c
@@ -788,14 +788,22 @@ 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;
 
+	lockdep_assert_held(&tb->lock);
+
 	if (!nhi->ops->reset_interface)
 		return;
 
-	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 +811,12 @@ static void tb_domain_reset_interface(struct tb *tb)
 	tb_ctl_start(tb->ctl);
 }
 
+static void tb_domain_reset_interface(struct tb *tb)
+{
+	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 +849,7 @@ 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);
-- 
2.43.0