On Wed, May 14, 2014 at 12:39:12PM -0700, John Fastabend wrote:

On 04/30/2014 09:35 AM, John Fastabend wrote:

quoted

Add __rcu notation to qdisc handling by doing this we can make
smatch output more legible. And anyways some of the cases should
be using rcu_dereference() see qdisc_all_tx_empty(),
qdisc_tx_chainging(), and so on.

Signed-off-by: John Fastabend <redacted>
---

Now I'm trying to resolve the lingering sparse errors/warnings and I
have one that I'm not sure about. Maybe someone has some insight,

net/sched/sch_generic.c:694:9: error: bad constant expression
net/sched/sch_generic.c:694:9: error: cannot size expression
net/sched/sch_generic.c:751:9: error: bad constant expression
net/sched/sch_generic.c:751:9: error: cannot size expression
net/sched/sch_generic.c:800:17: error: bad constant expression
net/sched/sch_generic.c:800:17: error: cannot size expression
net/sched/sch_generic.c:886:9: error: bad constant expression
net/sched/sch_generic.c:886:9: error: cannot size expression
net/sched/sch_generic.c:908:17: error: bad constant expression
net/sched/sch_generic.c:908:17: error: cannot size expression

There is some compiletime_assert_atomic_type() bustage that causes
errors like this.  There should be a fix on its way in.  Try making
compiletime_assert_atomic_type() be an empty macro, and if that works,
help is on the way.

							Thanx, Paul

Here are (what I believe are) the relevant changes to trigger it,

quoted

 include/linux/netdevice.h |   29 ++++-------------------------
 include/net/sch_generic.h |   29 +++++++++++++++++++++++------
 net/core/dev.c            |   45 +++++++++++++++++++++++++++++++++++++++++++--
 net/sched/sch_generic.c   |    4 ++--
 net/sched/sch_mqprio.c    |    6 ++++--
 net/sched/sch_teql.c      |    9 +++++----
 6 files changed, 81 insertions(+), 41 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index a803d79..4826988 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h

@@ -546,7 +546,7 @@ struct netdev_queue {
  * read mostly part
  */
 	struct net_device	*dev;
-	struct Qdisc		*qdisc;
+	struct Qdisc __rcu	*qdisc;

Add __rcu here,

quoted

 	struct Qdisc		*qdisc_sleeping;
 #ifdef CONFIG_SYSFS
 	struct kobject		kobj;

[...]

quoted

+++ b/net/sched/sch_generic.c

[...]

quoted

@@ -871,7 +871,7 @@ static void dev_init_scheduler_queue(struct net_device *dev,
 {
 	struct Qdisc *qdisc = _qdisc;

-	dev_queue->qdisc = qdisc;
+	rcu_assign_pointer(dev_queue->qdisc, qdisc);
 	dev_queue->qdisc_sleeping = qdisc;
 }

Then rcu_assign_pointer() operations throw the "bad constant expression"
and "cannot size expression" errors. The line numbers are a bit off from
the errors above due to some other improvements/fixes but this is line
886 from above sparse output.

I've done similar types of transformations before without sparse errors
so I thought I might see what changed. After doing a revert of

"rcu: Define rcu_assign_pointer() in terms of smp_store_release()"
commit 88c1863066ccfa456797e12c5d8b4631aa1ad0d0

Sparse no longer throws an error. Here is the diff of that commit for
reference

quoted

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 00a7fd6..4550f22 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h

@@ -44,7 +44,6 @@

#include <linux/debugobjects.h>
#include <linux/bug.h>
#include <linux/compiler.h>
-#include <asm/barrier.h>

extern int rcu_expedited; /* for sysctl */
#ifdef CONFIG_RCU_TORTURE_TEST

@@ -582,7 +581,12 @@ static inline void rcu_preempt_sleep_check(void)
 * please be careful when making changes to rcu_assign_pointer() and the
 * other macros that it invokes.
 */
-#define rcu_assign_pointer(p, v) smp_store_release(&p, RCU_INITIALIZER(v))
+#define rcu_assign_pointer(p, v) \
+       do { \
+               smp_wmb(); \
+               ACCESS_ONCE(p) = RCU_INITIALIZER(v); \
+       } while (0)
+

/**
 * rcu_access_pointer() - fetch RCU pointer with no dereferencing

At the moment I'm not seeing what changed that would cause the error.
Any ideas what I got wrong here?

Thanks!
John

-- 
John Fastabend         Intel Corporation