Re: [PATCH] IB/core: Make ib_mad_client_id atomic

[Jason Gunthorpe]

On Thu, Apr 19, 2018 at 11:55:55PM -0400, Doug Ledford wrote:
> On Wed, 2018-04-18 at 16:24 +0200, HÃkon 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?

But it is on the ib_register_mad_agent() path which is not a
performance path..

This actually looks like a genuine bug, why is it described only as
'confusing'? ib_register_mad_agent is callable from userspace, so at
least two userspace agents can race and get the same TID's.

TIDs need to be globally unique on the entire machine.

Jason