From mboxrd@z Thu Jan  1 00:00:00 1970
From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Subject: Re: [PATCHv2 5/6] xen: use ticket locks for spin locks
Date: Fri, 10 Apr 2015 13:58:38 -0400
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To: David Vrabel <dvrabel@cantab.net>, David Vrabel <david.vrabel@citrix.com>, xen-devel@lists.xenproject.org
Cc: Keir Fraser <keir@xen.org>, Tim Deegan <tim@xen.org>, Ian Campbell <ian.campbell@citrix.com>, Jan Beulich <jbeulich@suse.com>, Stefano Stabellini <stefano.stabellini@eu.citrix.com>
List-Id: xen-devel@lists.xenproject.org

On 04/10/2015 01:29 PM, David Vrabel wrote:
>
> On 10/04/2015 17:44, Boris Ostrovsky wrote:
>> On 04/10/2015 10:19 AM, David Vrabel wrote:
>>
>>> @@ -138,14 +138,17 @@ void spin_debug_disable(void)
>>>
>>>    void _spin_lock(spinlock_t *lock)
>>>    {
>>> +    spinlock_tickets_t tickets = { .tail = 1, };
>>>        LOCK_PROFILE_VAR;
>>>
>>>        check_lock(&lock->debug);
>>> -    while ( unlikely(!_raw_spin_trylock(&lock->raw)) )
>>> +    tickets.head_tail = xadd(&lock->tickets, tickets.head_tail);
>>> +    if ( tickets.tail != read_atomic(&lock->tickets.head) )
>>>        {
>>>            LOCK_PROFILE_BLOCK;
>>> -        while ( likely(_raw_spin_is_locked(&lock->raw)) )
>>> +        do {
>>>                cpu_relax();
>>> +        } while ( tickets.tail != read_atomic(&lock->tickets.head) );
>>>        }
>>
>> Why do you use both 'if' and 'while"? I.e. why not just
>>
>> while ( tickets.tail != read_atomic(&lock->tickets.head) )
>> {
>>      LOCK_PROFILE_BLOCK;
>>      cpu_relax();
>> }
> We need to only call LOCK_PROFILE_BLOCK once when we start blocking.

LOCK_PROFILE_BLOCK is a "one-shot" assignment. Once 'block' is set to 
NOW() it doesn't change anymore. So it will be updated only on the first 
iteration of the loop.

-boris