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Subject: Re: [PATCH v5 for-next 3/3] IB/core: Obtain subnet_prefix from cache
 in IB devices
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     linux-rdma@vger.kernel.org, linux-kernel@vger.kernel.org,
        dledford@redhat.com, haakon.bugge@oracle.com, leon@kernel.org
References: <20210616154509.1047-1-anand.a.khoje@oracle.com>
 <20210616154509.1047-4-anand.a.khoje@oracle.com>
 <20210621234913.GA2364052@nvidia.com>
 <012d6cd2-5167-ed81-80db-444fd2741ea8@oracle.com>
 <20210624175458.GR2371267@nvidia.com>
From:   Anand Khoje <anand.a.khoje@oracle.com>
Organization: Oracle Corporation
Message-ID: <5fd99709-8a90-e875-1ed2-74f5bcce6eec@oracle.com>
Date:   Fri, 25 Jun 2021 11:33:58 +0530
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On 6/24/2021 11:24 PM, Jason Gunthorpe wrote:
> On Wed, Jun 23, 2021 at 06:33:32PM +0530, Anand Khoje wrote:
>> On 6/22/2021 5:19 AM, Jason Gunthorpe wrote:
>>> On Wed, Jun 16, 2021 at 09:15:09PM +0530, Anand Khoje wrote:
>>>> @@ -1523,13 +1524,21 @@ static int config_non_roce_gid_cache(struct ib_device *device,
>>>>    	device->port_data[port].cache.lmc = tprops->lmc;
>>>>    	device->port_data[port].cache.port_state = tprops->state;
>>>> -	device->port_data[port].cache.subnet_prefix = tprops->subnet_prefix;
>>>> +	ret = rdma_query_gid(device, port, 0, &gid);
>>>> +	if (ret) {
>>>
>>> This is quite a bit different than just calling ops.query_gid() - why
>>> are you changing it? I'm not sure all the additional tests will pass,
>>> the 0 gid entry is not required to be valid..
>>>
>> Hi Jason,
>>
>> We have opted for rdma_query_gid(), as during ib_cache_update() the code
>> calls ops.query_gid() earlier in config_non_roce_gid_cache(), thereby
>> updating the value of GID in cache. We utilize this updated value, instead
>> of calling ops->query_gid() again.
> 
> Uhhhh, so just store the subnet prefix at that point then?
> 
> diff --git a/drivers/infiniband/core/cache.c b/drivers/infiniband/core/cache.c
> index c9e9fc81447e89..5c554ebd000e89 100644
> --- a/drivers/infiniband/core/cache.c
> +++ b/drivers/infiniband/core/cache.c
> @@ -1428,8 +1428,8 @@ int rdma_read_gid_l2_fields(const struct ib_gid_attr *attr,
>   }
>   EXPORT_SYMBOL(rdma_read_gid_l2_fields);
>   
> -static int config_non_roce_gid_cache(struct ib_device *device,
> -				     u32 port, int gid_tbl_len)
> +static int config_non_roce_gid_cache(struct ib_device *device, u32 port,
> +				     struct ib_port_attr *tprops)
>   {
>   	struct ib_gid_attr gid_attr = {};
>   	struct ib_gid_table *table;
> @@ -1441,7 +1441,7 @@ static int config_non_roce_gid_cache(struct ib_device *device,
>   	table = rdma_gid_table(device, port);
>   
>   	mutex_lock(&table->lock);
> -	for (i = 0; i < gid_tbl_len; ++i) {
> +	for (i = 0; i < tprops->gid_tbl_len; ++i) {
>   		if (!device->ops.query_gid)
>   			continue;
>   		ret = device->ops.query_gid(device, port, i, &gid_attr.gid);
> @@ -1452,6 +1452,8 @@ static int config_non_roce_gid_cache(struct ib_device *device,
>   			goto err;
>   		}
>   		gid_attr.index = i;
> +		tprops->subnet_prefix =
> +			be64_to_cpu(gid_attr.global.subnet_prefix);
>   		add_modify_gid(table, &gid_attr);
>   	}
>   err:
> @@ -1484,7 +1486,7 @@ ib_cache_update(struct ib_device *device, u32 port, bool update_gids,
>   
>   	if (!rdma_protocol_roce(device, port) && update_gids) {
>   		ret = config_non_roce_gid_cache(device, port,
> -						tprops->gid_tbl_len);
> +						tprops);
>   		if (ret)
>   			goto err;
>   	}
> 

Hi Jason,

Thanks for the response!

If the above change is to be made, there could arise a scenario in which:
  In case of a cache_update event, another application/module could try 
to call ib_query_port() and read subnet_prefix while the cache is still 
getting updated and the application/module could end up reading a stale 
value of subnet_prefix.

I have a few questions:
- How likely is it that an up and running Infiniband fabric would change 
the subnet_prefix?
- Is it possible that different GIDs in the gid_table will have 
different values of subnet_prefix?

>>> And I would much prefer things be re-organized so the cache can be
>>> valid sooner to adding this variable. What is the earlier call that is
>>> motivating this?
>>
>> During device load and when cache is yet to be updated, ib_query_port()
>> should have a mechanism to identify if the cache entry is valid or invalid
>> (uninitialized), we have added this variable just to ensure the validity of
>> cache.
> 
> Unless there is an actual user of ib_query_port() before
> config_non_roce_gid_cache() that I can't see, don't bother, returning
> 0 is fine.
> 
> Jason
> 

Hm! that makes sense, with the above change we wouldn't need to call 
device->ops.query_gid() from __ib_query_port() and can always read 
subnet_prefix using ib_get_cached_subnet_prefix(), if reading stale 
value during cache update event is not an issue.

Thanks,
Anand