Re: [PATCH 2/2] docs: Update RCU's hotplug requirements with a bit about design

[Joel Fernandes <joel@joelfernandes.org>]

On Fri, Oct 2, 2020 at 3:34 PM Paul E. McKenney <paulmck@kernel.org> wrote:
>
> On Tue, Sep 29, 2020 at 03:32:48PM -0400, Joel Fernandes wrote:
> > Hi Paul,
> >
> > On Tue, Sep 29, 2020 at 03:29:28PM -0400, Joel Fernandes (Google) wrote:
> > > RCU's hotplug design will help understand the requirements an RCU
> > > implementation needs to fullfill, such as dead-lock avoidance.
> > >
> > > The rcu_barrier() section of the "Hotplug CPU" section already talks
> > > about deadlocks, however the description of what else can deadlock other
> > > than rcu_barrier is rather incomplete.
> > >
> > > This commit therefore continues the section by describing how RCU's
> > > design handles CPU hotplug in a deadlock-free way.
> > >
> > > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > > ---
> > >  .../RCU/Design/Requirements/Requirements.rst  | 30 +++++++++++++++++--
> > >  1 file changed, 28 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst
> > > index 1ae79a10a8de..e0413aa989dd 100644
> > > --- a/Documentation/RCU/Design/Requirements/Requirements.rst
> > > +++ b/Documentation/RCU/Design/Requirements/Requirements.rst
> > > @@ -1929,8 +1929,10 @@ The Linux-kernel CPU-hotplug implementation has notifiers that are used
> > >  to allow the various kernel subsystems (including RCU) to respond
> > >  appropriately to a given CPU-hotplug operation. Most RCU operations may
> > >  be invoked from CPU-hotplug notifiers, including even synchronous
> > > -grace-period operations such as ``synchronize_rcu()`` and
> > > -``synchronize_rcu_expedited()``.
> > > +grace-period operations such as. However, the synchronous variants
> > > +(``synchronize_rcu()`` and ``synchronize_rcu_expedited()``) should not
> > > +from notifiers that execute via ``stop_machine()`` -- specifically those
> >
> > The "should not from notifiers" should be "should not be used from
> > notifiers" here. Sorry and hope you can fix it up.
>
> Thank you, and queued for further review.  How does the below look
> for a general fixup?

Looks great, thanks!

 -Joel



>
>                                                 Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit a93716177eeac726037828b28e6b1a45e828688a
> Author: Joel Fernandes (Google) <joel@joelfernandes.org>
> Date:   Tue Sep 29 15:29:28 2020 -0400
>
>     docs: Update RCU's hotplug requirements with a bit about design
>
>     The rcu_barrier() section of the "Hotplug CPU" section discusses
>     deadlocks, however the description of deadlocks other than those involving
>     rcu_barrier() is rather incomplete.
>
>     This commit therefore continues the section by describing how RCU's
>     design handles CPU hotplug in a deadlock-free way.
>
>     Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
>     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>
> diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst
> index 1ae79a1..98557fe 100644
> --- a/Documentation/RCU/Design/Requirements/Requirements.rst
> +++ b/Documentation/RCU/Design/Requirements/Requirements.rst
> @@ -1929,16 +1929,45 @@ The Linux-kernel CPU-hotplug implementation has notifiers that are used
>  to allow the various kernel subsystems (including RCU) to respond
>  appropriately to a given CPU-hotplug operation. Most RCU operations may
>  be invoked from CPU-hotplug notifiers, including even synchronous
> -grace-period operations such as ``synchronize_rcu()`` and
> -``synchronize_rcu_expedited()``.
> -
> -However, all-callback-wait operations such as ``rcu_barrier()`` are also
> -not supported, due to the fact that there are phases of CPU-hotplug
> -operations where the outgoing CPU's callbacks will not be invoked until
> -after the CPU-hotplug operation ends, which could also result in
> -deadlock. Furthermore, ``rcu_barrier()`` blocks CPU-hotplug operations
> -during its execution, which results in another type of deadlock when
> -invoked from a CPU-hotplug notifier.
> +grace-period operations such as (``synchronize_rcu()`` and
> +``synchronize_rcu_expedited()``).  However, these synchronous operations
> +do block and therefore cannot be invoked from notifiers that execute via
> +``stop_machine()``, specifically those between the ``CPUHP_AP_OFFLINE``
> +and ``CPUHP_AP_ONLINE`` states.
> +
> +In addition, all-callback-wait operations such as ``rcu_barrier()`` may
> +not be invoked from any CPU-hotplug notifier.  This restriction is due
> +to the fact that there are phases of CPU-hotplug operations where the
> +outgoing CPU's callbacks will not be invoked until after the CPU-hotplug
> +operation ends, which could also result in deadlock. Furthermore,
> +``rcu_barrier()`` blocks CPU-hotplug operations during its execution,
> +which results in another type of deadlock when invoked from a CPU-hotplug
> +notifier.
> +
> +Finally, RCU must avoid deadlocks due to interaction between hotplug,
> +timers and grace period processing. It does so by maintaining its own set
> +of books that duplicate the centrally maintained ``cpu_online_mask``,
> +and also by reporting quiescent states explictly when a CPU goes
> +offline.  This explicit reporting of quiescent states avoids any need
> +for the force-quiescent-state loop (FQS) to report quiescent states for
> +offline CPUs.  However, as a debugging measure, the FQS loop does splat
> +if offline CPUs block an RCU grace period for too long.
> +
> +An offline CPU's quiescent state will be reported either:
> +1.  As the CPU goes offline using RCU's hotplug notifier (``rcu_report_dead()``).
> +2.  When grace period initialization (``rcu_gp_init()``) detects a
> +    race either with CPU offlining or with a task unblocking on a leaf
> +    ``rcu_node`` structure whose CPUs are all offline.
> +
> +The CPU-online path (``rcu_cpu_starting()``) should never need to report
> +a quiescent state for an offline CPU.  However, as a debugging measure,
> +it does emit a warning if a quiescent state was not already reported
> +for that CPU.
> +
> +During the checking/modification of RCU's hotplug bookkeeping, the
> +corresponding CPU's leaf node lock is held. This avoids race conditions
> +between RCU's hotplug notifier hooks, the grace period initialization
> +code, and the FQS loop, all of which refer to or modify this bookkeeping.
>
>  Scheduler and RCU
>  ~~~~~~~~~~~~~~~~~