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From: Ian Munsie <imunsie@au1.ibm.com>
To: Vaibhav Jain <vaibhav@linux.vnet.ibm.com>
Cc: Philippe Bergheaud <felix@linux.vnet.ibm.com>,
 linuxppc-dev <linuxppc-dev@lists.ozlabs.org>, mikey <mikey@neuling.org>,
 "Matthew R. Ochs" <mrochs@linux.vnet.ibm.com>,
 manoj <manoj@linux.vnet.ibm.com>
Subject: Re: [v6,
 1/2] cxl: Add mechanism for delivering AFU driver specific events
In-reply-to: <87k2hkgs6v.fsf@vajain21.in.ibm.com>
References: <1466086415-30586-1-git-send-email-felix@linux.vnet.ibm.com>
 <87k2hkgs6v.fsf@vajain21.in.ibm.com>
Date: Tue, 21 Jun 2016 14:49:33 +1000
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Excerpts from Vaibhav Jain's message of 2016-06-20 14:20:16 +0530:
> > +int cxl_unset_driver_ops(struct cxl_context *ctx)
> > +{
> > +    if (atomic_read(&ctx->afu_driver_events))
> > +        return -EBUSY;
> > +
> > +    ctx->afu_driver_ops = NULL;
> Need a write memory barrier so that afu_driver_ops isnt possibly called
> after this store.

What situation do you think this will help? I haven't looked closely at
the last few iterations of this patch set, but if you're in a situation
where you might be racing with some code doing e.g.

if (ctx->afu_driver_ops)
	ctx->afu_driver_ops->something();

You have a race with or without a memory barrier. Ideally you would just
have the caller guarantee that it will only call cxl_unset_driver_ops if
no further calls to afu_driver_ops is possible, otherwise you may need
locking here which would be far from ideal.


What exactly is the use case for this API? I'd vote to drop it if we can
do without it.

-Ian