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SubjectRe: [PATCH] rcu,doc: lock-free update site
On 06/14/2011 08:50 PM, Mathieu Desnoyers wrote:
> * Lai Jiangshan (laijs@cn.fujitsu.com) wrote:
>> Add a document which describes a pattern of using RCU to implement lock-free(lockless)
>> update site.
>>
> [...]
>> @@ -0,0 +1,143 @@
>> +Lock-free(lockless) update site
>> +
>> +This article describes a pattern of using RCU to implement lock-free(lockless)
>> +update site. RCU update site is considered call-rare and it is protected
>> +by a update-site lock generally. But blocking algorithms are undesirable
>> +in some cases for some reasons, thus, this pattern may help.
>
> Hi Lai,
>
> Yes, using this kind of rcu read-side lock to protect against the
> cmpxchg ABA problem is well-known (to me at least) ;) I used this
> technique in the userspace RCU library "lock-free queue" and "lock-free
> stack" in 2010*. Please feel free to dig through my RCU data containers code
> to bring in more data structure examples:
>
> http://git.lttng.org/?p=userspace-rcu.git;a=blob;f=urcu/static/rculfqueue.h;h=b627e450cfdd581692b474d89437e3fd47f18463;hb=HEAD
>
> http://git.lttng.org/?p=userspace-rcu.git;a=blob;f=urcu/static/rculfqueue.h;h=b627e450cfdd581692b474d89437e3fd47f18463;hb=HEAD
>
> Thanks!
>
> Mathieu


Hi, Mathieu

I will try to make simple update site wait-free, so I wrote a simple guide/document
at first. I didn't notice your work. Your work is excellent,
I will add some references. I hope you rewrite/pretty this document also.

The lock-free stack is a good example, but the lock-free queue is not good here I think.

Thanks,
Lai


>
> * AFAIK I introduced this technique using RCU read-side C.S. to deal
> with cmpxchg ABA at that point, but someone might have thought about
> it before me without my knowledge. My litterature survey so far
> indicates that using a double-word CAS on a pointer/counter was one of
> the usual technique used to protect against cmpxchg ABA so far. Other
> techniques imply allocating elements in a limited-size array (so a
> simple cmpxchg can update the array index and counter atomically),
> Hasard Pointers, or having a full-blown GC which provides similar
> guarantees to the RCU grace period with a read-side lock held.
> Ref.:
>
> [1998] Maged Michael, Michael Scott "Simple, fast, and practical non-blocking and blocking concurrent queue algorithms"
> [2002] Maged M.Michael "Safe memory reclamation for dynamic lock-free objects using atomic reads and writes"
> [2003] Maged M.Michael "Hazard Pointers: Safe memory reclamation for lock-free objects"
>



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