linux-kernel - Re: rcu-bh design

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Date:   Fri, 4 May 2018 13:11:29 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Joel Fernandes <joel.opensrc@...il.com>
Cc:     Joel Fernandes <joelaf@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: rcu-bh design

On Fri, May 04, 2018 at 12:57:19PM -0700, Joel Fernandes wrote:
> On Fri, May 4, 2018 at 11:49 AM, Paul E. McKenney
> <paulmck@...ux.vnet.ibm.com> wrote:
> > On Fri, May 04, 2018 at 06:34:32PM +0000, Joel Fernandes wrote:
> >> On Fri, May 4, 2018 at 10:42 AM Paul E. McKenney
> >> <paulmck@...ux.vnet.ibm.com>
> >> wrote:
> >> [...]
> >> > > > > But preemptible RCU *does not* use context-switch as a quiescent
> >> state.
> >> > > > It doesn't?
> >> > >
> >> > > I thought that's what preemptible rcu is about. You can get preempted
> >> but
> >> > > you shouldn't block in a read-section. Is that not true?
> >>
> >> > Almost.  All context switches in an RCU-preempt read-side critical section
> >> > must be subject to priority boosting.  Preemption is one example, because
> >> > boosting the priority of the preempted task will make it runnable.
> >> > The priority-inheritance -rt "spinlock" is another example, because
> >> > boosting the priority of the task holding the lock will eventually make
> >> > runnable the task acquiring the lock within the RCU-preempt read-side
> >> > critical section.
> >>
> >> Yes I understand priority boosting is needed with preemptible RCU so that
> >> read-sections are making forward progress. I meant (and correct me if I'm
> >> wrong) that, as long as a task doesn't sleep in a preemptible RCU
> >> read-section (rcu-preempt flavor), then bad things wont happen and RCU will
> >> work correctly.
> >
> > The exception is -rt "spinlock" acquisition.  If the "spinlock" is held,
> > the task acquiring it will block, which is legal within an RCU-preempt
> > read-side critical section.
> >
> > This exception is why I define bad things in terms of lack of
> > susceptibility to priority boosting instead of sleeping.
> 
> Oh, that's a tricky situation. Thanks for letting me know. I guess my
> view was too idealistic. Makes sense now.

Well, let me put it this way...

Here is your nice elegant little algorithm:
https://upload.wikimedia.org/wikipedia/commons/6/6e/Golde33443.jpg

Here is your nice elegant little algorithm equipped to survive within
the Linux kernel:
https://totatema.files.wordpress.com/2012/06/feeling_grizzly-1600x1200.jpg

Any questions?  ;-)

							Thanx, Paul