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

[Paolo Bonzini]

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.

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...)