From: Alexander Sverdlin alexander.sverdlin@siemens.com

When LAN9303 is MDIO-connected two callchains exist into mdio->bus->write():

1. switch ports 1&2 ("physical" PHYs):

virtual (switch-internal) MDIO bus (lan9303_switch_ops->phy_{read|write})-> lan9303_mdio_phy_{read|write} -> mdiobus_{read|write}_nested

2. LAN9303 virtual PHY:

virtual MDIO bus (lan9303_phy_{read|write}) -> lan9303_virt_phy_reg_{read|write} -> regmap -> lan9303_mdio_{read|write}

If the latter functions just take mutex_lock(&sw_dev->device->bus->mdio_lock) it triggers a LOCKDEP false-positive splat. It's false-positive because the first mdio_lock in the second callchain above belongs to virtual MDIO bus, the second mdio_lock belongs to physical MDIO bus.

Consequent annotation in lan9303_mdio_{read|write} as nested lock (similar to lan9303_mdio_phy_{read|write}, it's the same physical MDIO bus) prevents the following splat:

WARNING: possible circular locking dependency detected 5.15.71 #1 Not tainted ------------------------------------------------------ kworker/u4:3/609 is trying to acquire lock: ffff000011531c68 (lan9303_mdio:131:(&lan9303_mdio_regmap_config)->lock){+.+.}-{3:3}, at: regmap_lock_mutex but task is already holding lock: ffff0000114c44d8 (&bus->mdio_lock){+.+.}-{3:3}, at: mdiobus_read which lock already depends on the new lock. the existing dependency chain (in reverse order) is: -> #1 (&bus->mdio_lock){+.+.}-{3:3}: lock_acquire __mutex_lock mutex_lock_nested lan9303_mdio_read _regmap_read regmap_read lan9303_probe lan9303_mdio_probe mdio_probe really_probe __driver_probe_device driver_probe_device __device_attach_driver bus_for_each_drv __device_attach device_initial_probe bus_probe_device deferred_probe_work_func process_one_work worker_thread kthread ret_from_fork -> #0 (lan9303_mdio:131:(&lan9303_mdio_regmap_config)->lock){+.+.}-{3:3}: __lock_acquire lock_acquire.part.0 lock_acquire __mutex_lock mutex_lock_nested regmap_lock_mutex regmap_read lan9303_phy_read dsa_slave_phy_read __mdiobus_read mdiobus_read get_phy_device mdiobus_scan __mdiobus_register dsa_register_switch lan9303_probe lan9303_mdio_probe mdio_probe really_probe __driver_probe_device driver_probe_device __device_attach_driver bus_for_each_drv __device_attach device_initial_probe bus_probe_device deferred_probe_work_func process_one_work worker_thread kthread ret_from_fork other info that might help us debug this: Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(&bus->mdio_lock); lock(lan9303_mdio:131:(&lan9303_mdio_regmap_config)->lock); lock(&bus->mdio_lock); lock(lan9303_mdio:131:(&lan9303_mdio_regmap_config)->lock); *** DEADLOCK *** 5 locks held by kworker/u4:3/609: #0: ffff000002842938 ((wq_completion)events_unbound){+.+.}-{0:0}, at: process_one_work #1: ffff80000bacbd60 (deferred_probe_work){+.+.}-{0:0}, at: process_one_work #2: ffff000007645178 (&dev->mutex){....}-{3:3}, at: __device_attach #3: ffff8000096e6e78 (dsa2_mutex){+.+.}-{3:3}, at: dsa_register_switch #4: ffff0000114c44d8 (&bus->mdio_lock){+.+.}-{3:3}, at: mdiobus_read stack backtrace: CPU: 1 PID: 609 Comm: kworker/u4:3 Not tainted 5.15.71 #1 Workqueue: events_unbound deferred_probe_work_func Call trace: dump_backtrace show_stack dump_stack_lvl dump_stack print_circular_bug check_noncircular __lock_acquire lock_acquire.part.0 lock_acquire __mutex_lock mutex_lock_nested regmap_lock_mutex regmap_read lan9303_phy_read dsa_slave_phy_read __mdiobus_read mdiobus_read get_phy_device mdiobus_scan __mdiobus_register dsa_register_switch lan9303_probe lan9303_mdio_probe ...

Cc: stable@vger.kernel.org Fixes: dc7005831523 ("net: dsa: LAN9303: add MDIO managed mode support") Signed-off-by: Alexander Sverdlin alexander.sverdlin@siemens.com --- Tested with v5.15 vendor kernel, compiled against net-next/main (cc54d2e2c58a). Affected code is unchanged since original introduction in 2017.

drivers/net/dsa/lan9303_mdio.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/dsa/lan9303_mdio.c b/drivers/net/dsa/lan9303_mdio.c index d8ab2b77d201..167a86f39f27 100644 --- a/drivers/net/dsa/lan9303_mdio.c +++ b/drivers/net/dsa/lan9303_mdio.c @@ -32,7 +32,7 @@ static int lan9303_mdio_write(void *ctx, uint32_t reg, uint32_t val) struct lan9303_mdio *sw_dev = (struct lan9303_mdio *)ctx;

reg <<= 2; /* reg num to offset */ - mutex_lock(&sw_dev->device->bus->mdio_lock); + mutex_lock_nested(&sw_dev->device->bus->mdio_lock, MDIO_MUTEX_NESTED); lan9303_mdio_real_write(sw_dev->device, reg, val & 0xffff); lan9303_mdio_real_write(sw_dev->device, reg + 2, (val >> 16) & 0xffff); mutex_unlock(&sw_dev->device->bus->mdio_lock); @@ -50,7 +50,7 @@ static int lan9303_mdio_read(void *ctx, uint32_t reg, uint32_t *val) struct lan9303_mdio *sw_dev = (struct lan9303_mdio *)ctx;

reg <<= 2; /* reg num to offset */ - mutex_lock(&sw_dev->device->bus->mdio_lock); + mutex_lock_nested(&sw_dev->device->bus->mdio_lock, MDIO_MUTEX_NESTED); *val = lan9303_mdio_real_read(sw_dev->device, reg); *val |= (lan9303_mdio_real_read(sw_dev->device, reg + 2) << 16); mutex_unlock(&sw_dev->device->bus->mdio_lock);