[lttng-dev] RCU Question

[lttng-dev] RCU Question

[siddharth teotia]

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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[lttng-dev] RCU Question

[siddharth teotia]

The mail was accidentally sent. Here is more info:

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 ?

Thanks,
Siddharth

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
>
>
>


-- 
*Best Regards,*
*SIDDHARTH TEOTIA*
*2008C6PS540G*
*BITS PILANI- GOA CAMPUS*

*+91 87911 75932*
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