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    SubjectRe: [PATCH tip/core/rcu 04/15] rcu: Permit RCU_NONIDLE() to be used from interrupt context
    On Thu, Aug 30, 2012 at 11:56:17AM -0700, Paul E. McKenney wrote:
    > From: "Paul E. McKenney" <paul.mckenney@linaro.org>
    >
    > There is a need to use RCU from interrupt context, but either before
    > rcu_irq_enter() is called or after rcu_irq_exit() is called. If the
    > interrupt occurs from idle, then lockdep-RCU will complain about such
    > uses, as they appear to be illegal uses of RCU from the idle loop.
    > In other environments, RCU_NONIDLE() could be used to properly protect
    > the use of RCU, but RCU_NONIDLE() currently cannot be invoked except
    > from process context.
    >
    > This commit therefore modifies RCU_NONIDLE() to permit its use more
    > globally.
    >
    > Reported-by: Steven Rostedt <rostedt@goodmis.org>
    > Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
    > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

    Something seems wrong about this. The addition of EXPORT_SYMBOL_GPL
    suggests that such interrupt handlers might live in modules. In what
    situation might a module interrupt handler get called from the idle
    loop, before rcu_irq_enter or after rcu_irq_exit, and need to know that
    when using RCU?

    - Josh Triplett

    > ---
    > include/linux/rcupdate.h | 6 ++----
    > kernel/rcutiny.c | 2 ++
    > kernel/rcutree.c | 2 ++
    > 3 files changed, 6 insertions(+), 4 deletions(-)
    >
    > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
    > index 115ead2..0fbbd52 100644
    > --- a/include/linux/rcupdate.h
    > +++ b/include/linux/rcupdate.h
    > @@ -210,14 +210,12 @@ extern void exit_rcu(void);
    > * to nest RCU_NONIDLE() wrappers, but the nesting level is currently
    > * quite limited. If deeper nesting is required, it will be necessary
    > * to adjust DYNTICK_TASK_NESTING_VALUE accordingly.
    > - *
    > - * This macro may be used from process-level code only.
    > */
    > #define RCU_NONIDLE(a) \
    > do { \
    > - rcu_idle_exit(); \
    > + rcu_irq_enter(); \
    > do { a; } while (0); \
    > - rcu_idle_enter(); \
    > + rcu_irq_exit(); \
    > } while (0)
    >
    > /*
    > diff --git a/kernel/rcutiny.c b/kernel/rcutiny.c
    > index e4163c5..2e073a2 100644
    > --- a/kernel/rcutiny.c
    > +++ b/kernel/rcutiny.c
    > @@ -115,6 +115,7 @@ void rcu_irq_exit(void)
    > rcu_idle_enter_common(newval);
    > local_irq_restore(flags);
    > }
    > +EXPORT_SYMBOL_GPL(rcu_irq_exit);
    >
    > /* Common code for rcu_idle_exit() and rcu_irq_enter(), see kernel/rcutree.c. */
    > static void rcu_idle_exit_common(long long oldval)
    > @@ -172,6 +173,7 @@ void rcu_irq_enter(void)
    > rcu_idle_exit_common(oldval);
    > local_irq_restore(flags);
    > }
    > +EXPORT_SYMBOL_GPL(rcu_irq_enter);
    >
    > #ifdef CONFIG_DEBUG_LOCK_ALLOC
    >
    > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
    > index f280e54..96b8aff 100644
    > --- a/kernel/rcutree.c
    > +++ b/kernel/rcutree.c
    > @@ -447,6 +447,7 @@ void rcu_irq_exit(void)
    > rcu_idle_enter_common(rdtp, oldval);
    > local_irq_restore(flags);
    > }
    > +EXPORT_SYMBOL_GPL(rcu_irq_exit);
    >
    > /*
    > * rcu_idle_exit_common - inform RCU that current CPU is moving away from idle
    > @@ -542,6 +543,7 @@ void rcu_irq_enter(void)
    > rcu_idle_exit_common(rdtp, oldval);
    > local_irq_restore(flags);
    > }
    > +EXPORT_SYMBOL_GPL(rcu_irq_enter);
    >
    > /**
    > * rcu_nmi_enter - inform RCU of entry to NMI context
    > --
    > 1.7.8
    >


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