linux-kernel - Re: [PATCH v5 for-next 3/3] IB/core: Obtain subnet

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Message-ID: <20210625124812.GS2371267@nvidia.com>
Date:   Fri, 25 Jun 2021 09:48:12 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Anand Khoje <anand.a.khoje@...cle.com>
Cc:     linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org,
        dledford@...hat.com, haakon.bugge@...cle.com, leon@...nel.org
Subject: Re: [PATCH v5 for-next 3/3] IB/core: Obtain subnet_prefix from cache
 in IB devices

On Fri, Jun 25, 2021 at 11:33:58AM +0530, Anand Khoje wrote:
> 
> 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.

Applications relying on this data must hook the event and update their
state when the event fires.

So long as ib_query_port returns the correct value in the event
handler it is all OK. This whole thing is racy - the HW can change the
subnet_prefix at anytime, this is just shuffling the race around.

> - Is it possible that different GIDs in the gid_table will have different
> values of subnet_prefix?

Valid GIDs should have the same prefix

Jason