-Also unlike other RCU flavors, SRCU's callbacks-wait function -srcu_barrier() may be invoked from CPU-hotplug notifiers, -though this is not necessarily a good idea. -The reason that this is possible is that SRCU is insensitive -to whether or not a CPU is online, which means that srcu_barrier() -need not exclude CPU-hotplug operations. +Also unlike other RCU flavors, synchronize_srcu() may not +be invoked from CPU-hotplug notifiers, due to the fact that SRCU grace +periods make use of timers and the possibility of timers being temporarily +“stranded” on the outgoing CPU. +This stranding of timers means that timers posted to the outgoing CPU +will not fire until late in the CPU-hotplug process. +The problem is that if a notifier is waiting on an SRCU grace period, +that grace period is waiting on a timer, and that timer is stranded on the +outgoing CPU, then the notifier will never be awakened, in other words, +deadlock has occurred. +This same situation of course also prohibits srcu_barrier() +from being invoked from CPU-hotplug notifiers.
SRCU also differs from other RCU flavors in that SRCU's expedited and