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SubjectRe: [PATCH 13/14] x86/ticketlock: add slowpath logic
On Tue, Nov 16, 2010 at 01:08:44PM -0800, Jeremy Fitzhardinge wrote:
> From: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
>
> Maintain a flag in both LSBs of the ticket lock which indicates whether
> anyone is in the lock slowpath and may need kicking when the current
> holder unlocks. The flags are set when the first locker enters
> the slowpath, and cleared when unlocking to an empty queue.
>
> In the specific implementation of lock_spinning(), make sure to set
> the slowpath flags on the lock just before blocking. We must do
> this before the last-chance pickup test to prevent a deadlock
> with the unlocker:
>
> Unlocker Locker
> test for lock pickup
> -> fail
> test slowpath + unlock
> -> false
> set slowpath flags
> block
>
> Whereas this works in any ordering:
>
> Unlocker Locker
> set slowpath flags
> test for lock pickup
> -> fail
> block
> test slowpath + unlock
> -> true, kick

I think this is still racy ..

Unlocker Locker


test slowpath
-> false

set slowpath flag
test for lock pickup
-> fail
block


unlock

unlock needs to happen first before testing slowpath? I have made that change
for my KVM guest and it seems to be working well with that change .. Will
cleanup and post my patches shortly

- vatsa


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