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 12:44:38 -0400
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To: David Vrabel <david.vrabel@citrix.com>, xen-devel@lists.xenproject.org
Cc: Tim Deegan <tim@xen.org>, Keir Fraser <keir@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 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();
}




>
> @@ -194,35 +174,44 @@ void _spin_unlock(spinlock_t *lock)
>   {
>       preempt_enable();
>       LOCK_PROFILE_REL;
> -    _raw_spin_unlock(&lock->raw);
> +    lock->tickets.head++;


Is it safe to do a plain increment here and a locked one?


-boris