[lttng-dev] RCU Question

siddharth teotia siddharthteotia at gmail.com
Fri Apr 29 19:07:41 UTC 2016

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
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

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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