[ltt-dev] [RFC PATCH 0/7] priority-boost urcu

Lai Jiangshan laijs at cn.fujitsu.com
Wed Aug 17 22:36:29 EDT 2011

On 08/17/2011 04:46 PM, Paolo Bonzini wrote:
> On 08/16/2011 12:58 AM, Lai Jiangshan wrote:
>> These series patches implelent a priority-boost urcu
>> based on pi-lock.
>> Some other locks(especial rcu_gp_lock) should be also
>> priority-aware, these patches did touch them and make
>> the patchset simpler.
> While really cool, I found this patchset overly complex.
> What we should introduce is abstractions over futexes. 

I think any general purpose abstractions over futexes should be included in pthread lib.
manual-reset event can/should be implemented over pthread API's.
But I don't want to add overhead for read site.

> This is what I did to experimentally port URCU to QEMU---my secret goal since commit 806f811 (use kernel style makefile output, 2010-03-01). :)  Our use of futexes is exceptionally similar to a Windows manual-reset event (yes, Windows: http://msdn.microsoft.com/en-us/library/system.threading.manualresetevent%28v=vs.80%29.aspx).  In QEMU I added the manual-reset event and use it in the implementation of RCU.
> By introducing an abstraction for this, we can make the code a lot clearer and secondarily gain in portability.  For QEMU portability was actually my primary goal, but URCU might have different priorities. :)
> PI futex support can also be implemented in the same framework.
> By the way, it is my impression that MB (perhaps MEMBARRIER too?) is way way more similar to QSBR than to SIGNAL:
>    MB rcu_read_unlock = QSBR rcu_thread_offline + nesting count
>    MB rcu_read_lock   = QSBR rcu_thread_online + nesting count
> Perhaps moving around code could make the code simpler?  Following the master/slave memory barrier functions is quite hard, and this is complicated by the KICK_READER_LOOPS that (if I understand correctly) makes little sense for non-SIGNAL models.
> Paolo

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