On Fri, Oct 2, 2020 at 3:34 PM Paul E. McKenney <paulmck@xxxxxxxxxx> 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@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? Looks great, thanks! -Joel > > 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 > ~~~~~~~~~~~~~~~~~
