Re: Preemptable Ticket Spinlock

[Jiannan Ouyang]

On Sun, Apr 21, 2013 at 5:12 PM, Rik van Riel <riel@xxxxxxxxxx> wrote:
> Your algorithm is very clever, and very promising.
>
> However, it does increase the size of the struct spinlock, and adds
> an additional atomic operation to spin_unlock, neither of which I
> suspect are necessary.
>
> If we always incremented the ticket number by 2 (instead of 1), then
> we could use the lower bit of the ticket number as the spinlock.
>
> If we do NOT run virtualized, we simply increment the ticket by 2
> in spin_unlock, and the code can remain otherwise the same.
>
> If we do run virtualized, we take that spinlock after acquiring
> the ticket (or timing out), just like in your code. In the
> virtualized spin_unlock, we can then release the spinlock and
> increment the ticket in one operation: by simply increasing the
> ticket by 1.
>
> In other words, we should be able to keep the overhead of this
> to an absolute minimum, and keep spin_unlock to be always the
> same cost it is today.
>
> --
> All rights reversed

Hi Rik,

Thanks for your feedback.

Yes I agree with you
- increase the size of struct spinlock is unnecessary
- your idea of utilize the lower bit and save one atomic operation
from unlock is cool!

I can come up with a updated patch soon.

--Jiannan
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