Re: 2.6.39-rc6-mmotm0506 - lockdep splat in RCU code on page fault

Paul E. McKenney
2011-05-10T16:22:06Z

On Tue, May 10, 2011 at 10:57:46AM +0200, Ingo Molnar wrote:
>
> * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
>
> > - raw_spin_lock_irqsave(&rnp->lock, flags);
> > - rnp->wakemask |= rdp->grpmask;
> > - raw_spin_unlock_irqrestore(&rnp->lock, flags);
> > + do {
> > + old = rnp->wakemask;
> > + new = old | rdp->grpmask;
> > + } while (cmpxchg(&rnp->wakemask, old, new) != old);
>
> Hm, isnt this an inferior version of atomic_or_long() in essence?
>
> Note that atomic_or_long() is x86 only, so a generic one would have to be
> offered too i suspect, atomic_cmpxchg() driven or so - which would look like
> the above loop.
>
> Most architectures could offer atomic_or_long() i suspect.

Is the following what you had in mind? This (untested) patch provides
only the generic function: if this is what you had in mind, I can put
together optimized versions for a couple of the architectures.

Thanx, Paul

------------------------------------------------------------------------

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index cc6c53a..e7c2e69 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -171,6 +171,9 @@ config ARCH_HAS_DEFAULT_IDLE
config ARCH_HAS_CACHE_LINE_SIZE
def_bool y

+config ARCH_HAS_ATOMIC_OR_LONG
+ def_bool X86_64
+
config HAVE_SETUP_PER_CPU_AREA
def_bool y

diff --git a/include/linux/atomic.h b/include/linux/atomic.h
index 96c038e..2fc3222 100644
--- a/include/linux/atomic.h
+++ b/include/linux/atomic.h
@@ -34,4 +34,17 @@ static inline int atomic_inc_not_zero_hint(atomic_t *v, int hint)
}
#endif

+#ifndef CONFIG_ARCH_HAS_ATOMIC_OR_LONG
+static inline void atomic_or_long(unsigned long *v1, unsigned long v2)
+{
+ unsigned long old;
+ unsigned long new;
+
+ do {
+ old = ACCESS_ONCE(*v1);
+ new = old | v2;
+ } while (cmpxchg(v1, old, new) != old);
+}
+#endif /* #ifndef CONFIG_ARCH_HAS_ATOMIC_OR_LONG */
+
#endif /* _LINUX_ATOMIC_H */
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 20c22c5..86f44a3 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -36,7 +36,7 @@
#include <linux/interrupt.h>
#include <linux/sched.h>
#include <linux/nmi.h>
-#include <asm/atomic.h>
+#include <linux/atomic.h>
#include <linux/bitops.h>
#include <linux/module.h>
#include <linux/completion.h>
@@ -1525,15 +1525,10 @@ static void rcu_cpu_kthread_setrt(int cpu, int to_rt)
*/
static void rcu_cpu_kthread_timer(unsigned long arg)
{
- unsigned long old;
- unsigned long new;
struct rcu_data *rdp = per_cpu_ptr(rcu_state->rda, arg);
struct rcu_node *rnp = rdp->mynode;

- do {
- old = rnp->wakemask;
- new = old | rdp->grpmask;
- } while (cmpxchg(&rnp->wakemask, old, new) != old);
+ atomic_or_long(&rnp->wakemask, rdp->grpmask);
invoke_rcu_node_kthread(rnp);
}