tree fc2b3c97cf4e9026636ec1e5d03939c520e81a41
parent e88b015529233812ac3be12cfc53f22e8cf94a47
author Paul E. McKenney <paulmck@kernel.org> 1602617963 -0700
committer Paul E. McKenney <paulmck@kernel.org> 1602617963 -0700

rcu: Prevent lockdep-RCU splats on lock acquisition/release

The rcu_cpu_starting() and rcu_report_dead() functions transition the
current CPU between online and offline state from an RCU perspective.
Unfortunately, this means that the rcu_cpu_starting() function's lock
acquisition and the rcu_report_dead() function's lock releases happen
while the CPU is offline from an RCU perspective, which can result in
lockdep-RCU splats about using RCU from an offline CPU.  In reality,
aside from the splats, both transitions are safe because a new grace
period cannot start until these functions release their locks.

But the false-positive splats nevertheless need to be eliminated.

This commit therefore uses sequence-count-like synchronization to forgive
use of RCU while RCU thinks a CPU is offline across the full extent of
the rcu_cpu_starting() and rcu_report_dead() function's lock acquisitions
and releases.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>