[lttng-dev] RCU Question

siddharth teotia siddharthteotia at gmail.com
Fri Apr 29 19:10:27 UTC 2016

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My question really is that when writing a user space application that
leverages RCU, how do I ensure that I am using the proper
flavor/implementation of RCU ? I don't think so I can go ahead with
non-preemptive RCU. And the user space paper does not really tell or
distinguish clearly as to which of the following -- QSBR, MB, BP, signal
can be used for preemptive environments and do not restrict the reader to
not block or not sleep ?


On Fri, Apr 29, 2016 at 12:07 PM, siddharth teotia <
siddharthteotia at gmail.com> wrote:

> Hi Guys,
> I recently started exploring RCU to build a lock free hash table. I have
> read quite a bit of literature from LWN on RCU, its usage, API etc. I have
> the following question for which I have had mixed answers online:
> Does the user space implementation of RCU liburcu take care of preemptive
> v/s non-preemptive flavors of RCU. From what I understand RCU can be
> broadly divided into:
> 1. Classic RCU - High Performance but suitable for non-preemptive
> environments.
> 2. Preemptible RCU - Permits the reader to block on a resource or the read
> side critical sections to be preempted. But still does not allow any kind
> of sleep(). Moreover there seems to be some sort of limitation on the
> nature of blocking that can happen within RCU read side critical section.
> 3. Sleepable RCU - Seems to me an augmentation of preemptible RCU.
> The paper that describes the user space implementation of RCU suggests 4
> different flavors -- QSBR, Memory Barrier based, Bullet Proof, and Signal
> based.
> I don't think so in my application I can afford to not have the readers
> block or even sleep. If the reader wants to take some orthogonal lock/latch
> on an entirely different resource, it might end waiting for it.
> And because

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