Re: [Xen-devel] [PATCHv6 1/3] xen: use ticket locks for spin locks

[Tim Deegan <tim@xxxxxxx>]

At 12:21 +0100 on 14 May (1431606100), David Vrabel wrote:
> Replace the byte locks with ticket locks.  Ticket locks are: a) fair;
> and b) peform better when contented since they spin without an atomic
> operation.
> 
> The lock is split into two ticket values: head and tail.  A locker
> acquires a ticket by (atomically) increasing tail and using the
> previous tail value.  A CPU holds the lock if its ticket == head.  The
> lock is released by increasing head.
> 
> spin_lock_irq() and spin_lock_irqsave() now spin with irqs disabled
> (previously, they would spin with irqs enabled if possible).  This is
> required to prevent deadlocks when the irq handler tries to take the
> same lock with a higher ticket.
> 
> Architectures need only provide arch_fetch_and_add() and two barriers:
> arch_lock_acquire_barrier() and arch_lock_release_barrier().
> 
> Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>

Reviewed-by: Tim Deegan <tim@xxxxxxx>

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