Re: [patch net-next v2 6/7] devlink: allow to call devl_param_driverinit_value_get() without holding instance lock

[Jiri Pirko <jiri@resnulli.us>]

Fri, Feb 10, 2023 at 08:41:27PM CET, jacob.e.keller@intel.com wrote:
>
>
>On 2/10/2023 2:01 AM, Jiri Pirko wrote:
>> From: Jiri Pirko <jiri@nvidia.com>
>> 
>> If the driver maintains following basic sane behavior, the
>> devl_param_driverinit_value_get() function could be called without
>> holding instance lock:
>> 
>> 1) Driver ensures a call to devl_param_driverinit_value_get() cannot
>>    race with registering/unregistering the parameter with
>>    the same parameter ID.
>> 2) Driver ensures a call to devl_param_driverinit_value_get() cannot
>>    race with devl_param_driverinit_value_set() call with
>>    the same parameter ID.
>> 3) Driver ensures a call to devl_param_driverinit_value_get() cannot
>>    race with reload operation.
>> 
>> By the nature of params usage, these requirements should be
>> trivially achievable. If the driver for some off reason
>> is not able to comply, it has to take the devlink->lock while
>> calling devl_param_driverinit_value_get().
>> 
>> Remove the lock assertion and add comment describing
>> the locking requirements.
>> 
>> This fixes a splat in mlx5 driver introduced by the commit
>> referenced in the "Fixes" tag.
>
>Just to clarify, is it possible for mlx5 to take the instance lock
>instead of these changes? I agree the improvements make sense here, but
>it seems like an alternative fix would be to grab the lock around the
>get function call in mlx5.

You are correct, yes.

>
>Either way, the assumptions here seem reasonable and the series makes sense.
>
>Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
>
>Thanks,
>Jake
>
>> 
>> Lore: https://lore.kernel.org/netdev/719de4f0-76ac-e8b9-38a9-167ae239efc7@amd.com/
>> Reported-by: Kim Phillips <kim.phillips@amd.com>
>> Fixes: 075935f0ae0f ("devlink: protect devlink param list by instance lock")
>> Signed-off-by: Jiri Pirko <jiri@nvidia.com>
>> Reviewed-by: Simon Horman <simon.horman@corigine.com>
>> Acked-by: Jakub Kicinski <kuba@kernel.org>
>> ---
>>  net/devlink/leftover.c | 13 +++++++++++--
>>  1 file changed, 11 insertions(+), 2 deletions(-)
>> 
>> diff --git a/net/devlink/leftover.c b/net/devlink/leftover.c
>> index 6d3988f4e843..9f0256c2c323 100644
>> --- a/net/devlink/leftover.c
>> +++ b/net/devlink/leftover.c
>> @@ -9628,14 +9628,23 @@ EXPORT_SYMBOL_GPL(devlink_params_unregister);
>>   *
>>   *	This function should be used by the driver to get driverinit
>>   *	configuration for initialization after reload command.
>> + *
>> + *	Note that lockless call of this function relies on the
>> + *	driver to maintain following basic sane behavior:
>> + *	1) Driver ensures a call to this function cannot race with
>> + *	   registering/unregistering the parameter with the same parameter ID.
>> + *	2) Driver ensures a call to this function cannot race with
>> + *	   devl_param_driverinit_value_set() call with the same parameter ID.
>> + *	3) Driver ensures a call to this function cannot race with
>> + *	   reload operation.
>> + *	If the driver is not able to comply, it has to take the devlink->lock
>> + *	while calling this.
>>   */
>>  int devl_param_driverinit_value_get(struct devlink *devlink, u32 param_id,
>>  				    union devlink_param_value *val)
>>  {
>>  	struct devlink_param_item *param_item;
>>  
>> -	lockdep_assert_held(&devlink->lock);
>> -
>>  	if (WARN_ON(!devlink_reload_supported(devlink->ops)))
>>  		return -EOPNOTSUPP;
>>