Re: [PATCH] RDMA/device: Fix a race between mad_client and cm_client init

Ding Hui
2024-01-06T02:20:51Z

On 2024/1/4 20:37, Jason Gunthorpe wrote:
> On Thu, Jan 04, 2024 at 02:48:14PM +0800, Shifeng Li wrote:
>
>> The root cause is that mad_client and cm_client may init concurrently
>> when devices_rwsem write semaphore is downgraded in enable_device_and_get() like:
>
> That can't be true, the module loader infrastructue ensures those two
> things are sequential.
>

Please consider the sequence again and notice that:

1. We agree that dependencies ensure mad_client be registered before cm_client.
2. But the mad_client.add() is not invoked in ib_register_client(), since
there is no DEVICE_REGISTERED device at that time.
Instead, it will be delayed until the device driver init (e.g. mlx5_core)
in enable_device_and_get().
3. The ib_cm and mlx5_core can be loaded concurrently, after setting DEVICE_REGISTERED
and downgrade_write(&devices_rwsem) in enable_device_and_get(), there is a chance
that cm_client.add() can be invoked before mad_client.add().

T1(ib_core init) | T2(device driver init) | T3(ib_cm init)
---------------------------------------------------------------------------------------------------
ib_register_client mad_client
assign_client_id
add clients CLIENT_REGISTERED
(with clients_rwsem write)
down_read(&devices_rwsem);
xa_for_each_marked (&devices, DEVICE_REGISTERED)
nop # no devices
up_read(&devices_rwsem);

ib_register_device
enable_device_and_get
down_write(&devices_rwsem);
set DEVICE_REGISTERED
downgrade_write(&devices_rwsem);
ib_register_client cm_client
assign_client_id
add clients CLIENT_REGISTERED
(with clients_rwsem write)
down_read(&devices_rwsem);
xa_for_each_marked (&devices, DEVICE_REGISTERED)
add_client_context
down_write(&device->client_data_rwsem);
get CLIENT_DATA_REGISTERED
downgrade_write(&device->client_data_rwsem);
cm_client.add
cm_add_one
ib_register_mad_agent
ib_get_mad_port
__ib_get_mad_port return NULL!
set CLIENT_DATA_REGISTERED
up_read(&device->client_data_rwsem);
up_read(&devices_rwsem);
down_read(&clients_rwsem);
xa_for_each_marked (&clients, CLIENT_REGISTERED)
add_client_context [mad]
mad_client.add
add_client_context [cm]
nop # already CLIENT_DATA_REGISTERED
up_read(&clients_rwsem);
up_read(&devices_rwsem);

> You are trying to say that the post-client fixup stuff will still see
> the DEVICE_REGISTERED before it reaches the clients_rwsem lock?
>
> That probably just says the clients_rwsem should be obtained before
> changing the DEVICE_STATE too :\
>

--
Thanks,
- Ding Hui