* Frederic Weisbecker <[email protected]> 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 <[email protected]>
> Signed-off-by: Frederic Weisbecker <[email protected]>
> Cc: Paul E. McKenney <[email protected]>
> Cc: Ingo Molnar <[email protected]>
> Cc: Peter Zijlstra <[email protected]>
> Cc: Stable <[email protected]>
> ---
> 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() \
Very nice!
Note that a cherry-picked version of the manual revert from Paul is upstream
now with the main body of RCU changes, so we can do the finegrained split-up
queue approach for this one problematic commit (that is no longer problematic).
I think it can still go in for v2.6.40. Paul, what do you think?
Thanks,
Ingo
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