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

[Paul E. McKenney]

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@xxxxxxxxxxxxxxxxx>
> > ---
> >  .../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?

						Thanx, Paul

------------------------------------------------------------------------

commit a93716177eeac726037828b28e6b1a45e828688a
Author: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
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@xxxxxxxxxxxxxxxxx>
    Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

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
 ~~~~~~~~~~~~~~~~~