Re: [PATCH 00/14] PV ticket locks without expanding spinlock

[Avi Kivity <avi@redhat.com>]

On 11/16/2010 11:08 PM, Jeremy Fitzhardinge wrote:
> From: Jeremy Fitzhardinge<jeremy.fitzhardinge@citrix.com>
>
> Hi all,
>
> This is a revised version of the pvticket lock series.
>
> The early part of the series is mostly unchanged: it converts the bulk
> of the ticket lock code into C and makes the "small" and "large"
> ticket code common.  The only changes are the incorporation of various
> review comments.
>
> The latter part of the series converts from pv spinlocks to pv ticket
> locks (ie, using the ticket lock fastpath as-is, but adding pv ops for
> the ticketlock slowpaths).
>
> The significant difference here is that rather than adding a new
> ticket_t-sized element to arch_spinlock_t - effectively doubling the
> size - I steal the LSB of the tickets themselves to store a bit.  This
> allows the structure to remain the same size, but at the cost of
> halving the max number of CPUs (127 for a 8-bit ticket, and a hard max
> of 32767 overall).
>
> The extra bit (well, two, but one is unused) in indicates whether the
> lock has gone into "slowpath state", which means one of its lockers
> has entered its slowpath and has blocked in the hypervisor.  This
> means the current lock-holder needs to make sure it gets kicked out of
> the hypervisor on unlock.
>
> The spinlock remains in slowpath state until the last unlock happens
> (ie there are no more queued lockers).
>
> This code survives for a while with moderate testing, (make -j 100 on
> 8 VCPUs on a 4 PCPU system), but locks up after about 20 iterations,
> so there's still some race/deadlock in there (probably something
> misordered), but I think the basic approach is sound.

This is going to be very useful for kvm; I'd like to see the fixed 
version gets merged.

-- 
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