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Date: Thu, 13 Feb 2014 10:25:52 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] [RFC] Convert Qemu Timer List and Active
 Timers to RCU
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To: Alex Bligh <alex@alex.org.uk>, Mike Day <ncmike@ncultra.org>
Cc: peter.maydell@linaro.org, qemu-devel@nongnu.org

Il 13/02/2014 10:11, Alex Bligh ha scritto:
> Mike,
>
> On 12 Feb 2014, at 19:09, Mike Day wrote:
>
>> Allow readers to use RCU when reading Qemu timer lists. Applies to
>> Paolo Bonzini's RCU branch, https://github.com/bonzini/qemu/tree/rcu.
>>
>> This patch is for comment and review only.  The rcu branch needs to be
>> rebased on upstream.
>
> I'll certainly have a look through this. However before I do, what
> problem is this trying to solve? Do we think there is possibility
> of contention on the active timers lock? I used to think this was
> taken (let alone contented) relatively infrequently, but Rob Herring's
> recent email suggests to me the list is being modified in some
> circumstances rather more frequently than I thought.

I think that, more than contention, it tries to reduce the cost of 
synchronization primitives, especially the locking and unlocking of the 
list around the invocation of timer callbacks.

Paolo