Re: Correct use of ap->lock versus ap->host->lock ?

[Jeff Garzik <jgarzik@pobox.com>]

Mark Lord wrote:
> Jeff Garzik wrote:
>> Mark Lord wrote:
>>> Jeff / Tejun / Alan,
>>>
>>> I'm trying to sort out the spinlocks in sata_mv.
>>>
>>> In some places, the existing code uses ap->lock.
>>> But in others, notably the interrupt handling, it uses ap->host->lock.
>>>
>>> This looks buggy to me, and I'm wondering how to make it bulletproof.
>>
>> Look closely, there is only one lock.  ata_port does not have a 
>> spinlock, just a pointer...
> ..
> 
> Ahh.. in ata_port_alloc().  Thanks.
> 
> Mmmm... so this reduces potential parallelism in libata,
> meaning we could probably achieve better SMP performance
> if the ap->locks were unique for each port.
> 
> But at the expense of very tricky and difficult coding
> around shared host resources.
> 
> Not worth it today for spinning media, but this could be
> a big limitation for solid-state media in the near future.

Its questionable whether it is worth it even for RAM-based ATA devices 
like gigabyte i-Ram.

The only thing being locked is software state involved in submission and 
completion (either host-wide or port-wide) and a couple register writes, 
which is a very tiny piece of the whole puzzle.

You have a long, long list of bottlenecks before you ever get there...

	Jeff