On Fri, Aug 07, 2020 at 07:10:38PM -0700, Randy Dunlap wrote: > Hi-- > > On 8/7/20 10:07 AM, 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> Thank you Randy for pointing these out. I will fix the nits in the next revision. Regards, - Joel > > .../RCU/Design/Requirements/Requirements.rst | 22 +++++++++++++++++++ > > 1 file changed, 22 insertions(+) > > > > diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst > > index 16c64a2eff93..0a4148b9f743 100644 > > --- a/Documentation/RCU/Design/Requirements/Requirements.rst > > +++ b/Documentation/RCU/Design/Requirements/Requirements.rst > > @@ -1940,6 +1940,28 @@ 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. > > > > +Also, RCU's implementation avoids serious deadlocks which could occur due to > > +interaction between hotplug, timers and grace period processing. It does so by > > +maintaining its own bookkeeping of every CPU's hotplug state, independent of > > +the various CPU masks and by reporting quiescent states at explicit points. It > > +may come across as a surprise, but the force quiescent state loop (FQS) does > > +not report quiescent states for offline CPUs and is not required to. > > + > > +For an offline CPU, the quiescent state will be reported in either of: > > +1. During CPU offlining, using RCU's hotplug notifier (``rcu_report_dead()``). > > note, uses (), which is good: () > > > +2. During grace period initialization (``rcu_gp_init``) if it detected a race > > add for consistency & readability: rcu_gp_init() > > > + with CPU offlining, or a race with a task unblocking on a node which > > + previously had all of its CPUs offlined. > > + > > +The CPU onlining path (``rcu_cpu_starting``) does not need to a report > > ditto: rcu_cpu_starting() > > > +quiescent state for an offline CPU in fact it would trigger a warning if a > > Missing something; maybe like so: > > for an offline CPU; in fact > > > +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, grace period initialization code and the FQS loop > > +which can concurrently refer to or modify the bookkeeping. > > + > > Scheduler and RCU > > ~~~~~~~~~~~~~~~~~ > > > > > > cheers. > -- > ~Randy >
