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    SubjectRe: [PATCH] rcu: Fix unpaired rcu_irq_enter() from locking selftests
    On Fri, May 20, 2011 at 02:09:54AM +0200, Frederic Weisbecker wrote:
    > HARDIRQ_ENTER() maps to irq_enter() which calls rcu_irq_enter().
    > But HARDIRQ_EXIT() maps to __irq_exit() which doesn't call
    > rcu_irq_exit().
    >
    > So for every locking selftest that simulates hardirq disabled,
    > we create an imbalance in the rcu extended quiescent state
    > internal state.
    >
    > As a result, after the first missing rcu_irq_exit(), subsequent
    > irqs won't exit any extended quiescent state the time of the
    > interrupt servicing. Rcu read side critical section inside irqs
    > on that CPU won't be guaranteed anymore.
    >
    > To fix this, just use __irq_enter() to simulate the hardirq
    > context. This is sufficient for the locking selftests as we
    > don't need to exit any extended quiescent state or perform
    > any check that irqs normally do when they wake up from idle.
    >
    > As a side effect, this patch should make it possible to
    > restore
    > "rcu: Decrease memory-barrier usage based on semi-formal proof",
    > which eventually helped finding this bug.
    >
    > Reported-and-tested-by: Yinghai Lu <yinghai@kernel.org>
    > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
    > Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

    You know, I looked at this code, but failed to see __irq_exit() vs
    irq_enter()...

    Good catch, Frederic, and thanks to Yinghai and Ingo for all the
    testing!

    Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

    > Cc: Ingo Molnar <mingo@elte.hu>
    > Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    > Cc: Stable <stable@kernel.org>
    > ---
    > lib/locking-selftest.c | 2 +-
    > 1 files changed, 1 insertions(+), 1 deletions(-)
    >
    > diff --git a/lib/locking-selftest.c b/lib/locking-selftest.c
    > index 619313e..507a22f 100644
    > --- a/lib/locking-selftest.c
    > +++ b/lib/locking-selftest.c
    > @@ -144,7 +144,7 @@ static void init_shared_classes(void)
    >
    > #define HARDIRQ_ENTER() \
    > local_irq_disable(); \
    > - irq_enter(); \
    > + __irq_enter(); \
    > WARN_ON(!in_irq());
    >
    > #define HARDIRQ_EXIT() \
    > --
    > 1.7.3.2
    >
    > --
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