Re: [PATCH 06/10] device-core: use atomic_set on .realized property

From: Maxim Levitsky <mlevitsk@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>, qemu-devel@nongnu.org
Cc: stefanha@redhat.com
Subject: Re: [PATCH 06/10] device-core: use atomic_set on .realized property
Date: Wed, 30 Sep 2020 21:03:27 +0300	[thread overview]
Message-ID: <04df3e246ceeefbef0d832841b87fcfcb1baa9ad.camel@redhat.com> (raw)
In-Reply-To: <e2cf2854-72bc-2f17-88b8-a2548f184583@redhat.com>

On Wed, 2020-09-30 at 19:44 +0200, Paolo Bonzini wrote:
> On 30/09/20 16:31, Maxim Levitsky wrote:
> > > +
> > > +        qatomic_set(&dev->realized, value);
> > > +        /*
> > > +         * Ensure that concurrent users see this update prior to
> > > +         * any other changes done by unrealize.
> > > +         */
> > > +        smp_wmb();
> > 
> > I''l probably never fully understand where to use read/write/full barrier.
> > If I understand corrctly, read barrier prevents reads done by this thread to be reordered,
> > by the CPU and write barrier prevents writes done by this CPU to be re-ordered.
> 
> I must say that the above is not really satisfactory.  The right thing
> to do would be to say which changes are done by unrealize; then you
> should make sure that *after* reading something that unrealize could
> undo you check if dev->realized is still true.
I didn't fully understand this to be honest.

I just wanted to explain what I know and what I don't know about hardware barriers.

I know that read barriers should be paired with write barriers, like if one CPU has a write barrier,
which ensures the orders of writes to two memory locations, the other CPU can then use a read barrier to ensure
that it sees those writes in this order.

I thus think that reads of dev->realized should be paired with read barrier, 
but here a full memory barrier isn't really needed.

Best regards,
	Maxim Levitsky

> 
> scsi_device_find is one such case, but I'm not convinced it is enough.
> 
> Paolo
> 
> > Both (depending on the macro) usually imply compiler barrier (to avoid compilier re-ordering
> > stuff...)