linux-kernel - Tasks RCU vs Preempt RCU

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Message-ID: <20180518183623.GA163151@joelaf.mtv.corp.google.com>
Date:   Fri, 18 May 2018 11:36:23 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     paulmck@...ux.vnet.ibm.com, rostedt@...dmis.org,
        byungchul.park@....com, mathieu.desnoyers@...icios.com,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
Cc:     kernel-team@...roid.com
Subject: Tasks RCU vs Preempt RCU

Hi,

I was thinking about tasks-RCU and why its needed. Since preempt-RCU allows
tasks to be preempted in read-sections, can we not just reuse that mechanism
for the trampolines since we track all preempted tasks so we would wait on
all tasks preempted within a trampoline?

I am trying to understand what will _not_ work if we did that.. I'm guessing
the answer is that that would mean the trampoline has to be wrapped with
rcu_read_{lock,unlock} which may add some overhead, but please let me know
if I'm missing something else..

The advantage I guess is possible elimination of an RCU variant, and also
possibly eliminating the tasks RCU thread that monitors.. Anyway I was
thinking more in terms of the effort of reduction of the RCU flavors etc and
reducing complexity ideas.

thanks!

- Joel