From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Vrabel <david.vrabel@citrix.com>
Subject: Re: [PATCHv4 5/8] xen: use ticket locks for spin locks
Date: Mon, 11 May 2015 14:13:49 +0100
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To: Jan Beulich <JBeulich@suse.com>, David Vrabel <david.vrabel@citrix.com>
Cc: Keir Fraser <keir@xen.org>, Ian Campbell <ian.campbell@citrix.com>, Andrew Cooper <andrew.cooper3@citrix.com>, Jennifer Herbert <jennifer.herbert@citrix.com>, Tim Deegan <tim@xen.org>, xen-devel@lists.xenproject.org
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On 08/05/15 10:36, Jan Beulich wrote:
>>
>> +        }
>> +    }
>>      smp_mb();
>>  }
> 
> The old code had smp_mb() before _and_ after the check - is it really
> correct to drop the one before (or effectively replace it by smp_rmb()
> in observe_{lock,head}())?

Typical usage is:

    d->is_dying = DOMDYING_dying;
    spin_barrier(&d->domain_lock);

So there needs to be a barrier before we check that the lock is
released.  i.e., I removed the wrong smp_mb().

I don't see the need for the second barrier since there's no stores in
_spin_barrier() and observe_lock() and observe_head() both have their
required read barriers.

David