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Subject[PATCH -tip/core/rcu 1/6] Cleanups and fixes for RCU in face of heavy CPU-hotplug stress
This patch set provides the following cleanups and fixes for RCU in
workloads that involve lots of CPU hotplugging. Ingo, please reset
core/rcu to 240ebbf before applying these patches.

o Split Hierarchical RCU initialization into boot-time and
CPU-hotplug pieces, so that other CPUs never see the data
structures in the act of being stitched together.

o Introduce a cpu_notifier() that works as does the current
hotcpu_notifier(), but also notifies during boot time even
if !HOTPLUG_CPU.

o Make RCU use the new cpu_notifier() to simplify hotplug code
and reduce the number of notifiers in use. (This also fixes
the problem Ingo and Hugh found.)

o Preemptable RCU had a bug due to its overly clever attempt
to move counts from CPUs going offline. This is "unsafe
at any speed", so remove the cleverness.

o Since offline CPUs now can have non-zero counts, they must
now be included in the tracing.

o Fix typo in preemptable RCU's rcu_irq_enter() comment
submitted by Josh Triplett.

These pass tests combining rcutorture and continuous CPU-hotplug
operations, in other words, no sleeping between successive CPU-hotplug
operations. This set does not fix the bug involving deadlocks through
rcutorture, kthread_stop(), and migration threads. However, this bug
does not affect production systems, so should not stand in the way of
applying these patches.

Thanx, Paul

b/include/linux/cpu.h | 18 +++++++++-----
b/kernel/rcupdate.c | 16 ++++++++++++-
b/kernel/rcupreempt.c | 25 +-------------------
b/kernel/rcupreempt_trace.c | 6 ++--
b/kernel/rcutree.c | 54 ++++++++++++++++++++++++++------------------
kernel/rcupreempt.c | 12 +--------
kernel/rcutree.c | 18 ++++----------
7 files changed, 72 insertions(+), 77 deletions(-)

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