linux-kernel - Re: [PATCH] IB/core: Make ib_mad_client

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Date:   Thu, 19 Apr 2018 23:55:55 -0400
From:   Doug Ledford <dledford@...hat.com>
To:     Håkon Bugge <Haakon.Bugge@...cle.com>,
        Don Hiatt <don.hiatt@...el.com>,
        Dasaratharaman Chandramouli 
        <dasaratharaman.chandramouli@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Sean Hefty <sean.hefty@...el.com>
Cc:     linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] IB/core: Make ib_mad_client_id atomic

On Wed, 2018-04-18 at 16:24 +0200, Håkon Bugge wrote:
> Two kernel threads may get the same value for agent.hi_tid, if the
> agents are registered for different ports. As of now, this works, as
> the agent list is per port.
> 
> It is however confusing and not future robust. Hence, making it
> atomic.
> 

People sometimes underestimate the performance penalty of atomic ops. 
Every atomic op is the equivalent of a spin_lock/spin_unlock pair.  This
is why two atomics are worse than taking a spin_lock, doing what you
have to do, and releasing the spin_lock.  Is this really what you want
for a "confusing, let's make it robust" issue?

-- 
Doug Ledford <dledford@...hat.com>
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