linux-kernel - [PATCH 0/2] Working around CPU hotplug and static keys locking

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Message-Id: <20170727093726.30266-1-marc.zyngier@arm.com>
Date:   Thu, 27 Jul 2017 10:37:24 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Leo Yan <leo.yan@...aro.org>
Subject: [PATCH 0/2] Working around CPU hotplug and static keys locking

Since f2545b2d4ce1 ("jump_label: Reorder hotplug lock and
jump_label_lock"), it has become impossible to switch a static key
from a CPU hotplug notifier:

- On the primary CPU, cpu_hotplug_lock is taken by
  __cpuhp_setup_state(), and then again by static_key_slow_inc(). The
  lock being taken as a reader, so it is OK so far.

- On a secondary CPU, _cpu_up takes the lock *as a writer* on the boot
  CPU, and the secondary tries to switch the static key, taking the
  lock as well (as a reader). In that case, we're toasted.

I couldn't find an elegant solution to this, so this series works
around the issue in the most disgusting way, adding a _nolock version
of the static key API to be used in CPU hotplug situations.

I hate it. Really. Please do not apply this series, and suggest
something less ugly instead.

The second patch uses this API to work around the issue that Leo
reported, where the static key flipped on a secondary CPU brings the
box down in flames.

Marc Zyngier (2):
  jump_label: Introduce a _nolock version of the static key API
  clocksource/arm_arch_timer: Use static_branch_enable_nolock()

 Documentation/static-keys.txt        | 19 +++++++++
 drivers/clocksource/arm_arch_timer.c |  6 ++-
 include/linux/jump_label.h           | 12 ++++++
 kernel/jump_label.c                  | 82 +++++++++++++++++++++++++++++-------
 4 files changed, 103 insertions(+), 16 deletions(-)

-- 
2.11.0