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

[Jason Gunthorpe <jgg@ziepe.ca>]

On Sat, Jan 06, 2024 at 10:12:17AM +0800, Ding Hui wrote:
> 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);

Take the draft I sent previously and use down_write(&devices_rwsem) in
ib_register_client()

Jason