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SubjectRe: [GIT PULL rcu/urgent] Fix for RCU-related hang

* Paul E. McKenney <> wrote:

> Hello, Ingo,
> This series has a single patch that fixes a system hang that can occur
> in perhaps unusual but very real circumstances. This hang occurs
> because of a very stupid bug of mine introduced in commit b1420f1c
> (Make rcu_barrier() less disruptive) that can cause CPUs to miscount
> RCU callbacks. The sequence of events leading to the hang is as follows:
> 1. A CPU miscounts its callbacks.
> 2. That CPU invokes all of its callbacks, so that its callback
> list is empty, but the callback count is nonzero.
> 3. That CPU goes offline. Because its callback list is empty,
> RCU's CPU-hotplug CPU_DEAD notifiers leave both the list and
> the count alone. (In contrast, had the list been non-empty,
> RCU's CPU_DEAD notifiers would have emptied the list and
> zeroed the count.)
> 4. One of the remaining CPUs executes one of the rcu_barrier()
> family of primitives. The rcu_barrier() primitive notes
> that the offline CPU has a non-zero count of callbacks, and
> therefore hangs waiting for this count to reach zero. The
> theory behind the indefinite wait is that the only reason that
> an offline CPU can have a non-zero number of RCU callbacks is
> that the CPU's CPU_DEAD notifiers have not yet executed.
> But they already have executed, so the offlined CPU's callback
> count will remain non-zero until it is brought back online,
> in other words, perhaps never.
> However, this bug is likely to pass a combined rcutorture/CPU-hotplug
> stress test because offlined CPUs tend to be brought back online
> reasonably quickly. For the rcutorture tests to fail, the system must be
> in the state indicated by step #3 above at the time the "rmmod rcutorture"
> executes.
> The fix is simply to prevent the miscounting.
> This change is available in the git repository at:
> git:// rcu/urgent
> Thanx, Paul
> ------------------------------------------------------------------------
> Paul E. McKenney (1):
> rcu: Stop rcu_do_batch() from multiplexing the "count" variable
> kernel/rcutree.c | 14 +++++++-------
> 1 files changed, 7 insertions(+), 7 deletions(-)

Pulled, thanks Paul!


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