Extend notifier_call_chain to count nr_calls made

Since 2.6.18-something, the community has been bugged by the problem to
provide a clean and a stable mechanism to postpone a cpu-hotplug event as
lock_cpu_hotplug was badly broken.

This is another proposal towards solving that problem.  This one is along the
lines of the solution provided in kernel/workqueue.c

Instead of having a global mechanism like lock_cpu_hotplug, we allow the
subsytems to define their own per-subsystem hot cpu mutexes.  These would be
taken(released) where ever we are currently calling
lock_cpu_hotplug(unlock_cpu_hotplug).

Also, in the per-subsystem hotcpu callback function,we take this mutex before
we handle any pre-cpu-hotplug events and release it once we finish handling
the post-cpu-hotplug events.  A standard means for doing this has been
provided in [PATCH 2/4] and demonstrated in [PATCH 3/4].

The ordering of these per-subsystem mutexes might still prove to be a
problem, but hopefully lockdep should help us get out of that muddle.

The patch set to be applied against linux-2.6.19-rc5 is as follows:

[PATCH 1/4] :	Extend notifier_call_chain with an option to specify the
		number of notifications to be sent and also count the
		number of notifications actually sent.

[PATCH 2/4] :	Define events CPU_LOCK_ACQUIRE and CPU_LOCK_RELEASE
		and send out notifications for these in _cpu_up and
		_cpu_down. This would help us standardise the acquire and
		release of the subsystem locks in the hotcpu
		callback functions of these subsystems.

[PATCH 3/4] :	Eliminate lock_cpu_hotplug from kernel/sched.c.

[PATCH 4/4] :	In workqueue_cpu_callback function, acquire(release) the
		workqueue_mutex while handling
		CPU_LOCK_ACQUIRE(CPU_LOCK_RELEASE).

If the per-subsystem-locking approach survives the test of time, we can expect
a slow phasing out of lock_cpu_hotplug, which has not yet been eliminated in
these patches :)

This patch:

Provide notifier_call_chain with an option to call only a specified number of
notifiers and also record the number of call to notifiers made.

The need for this enhancement was identified in the post entitled
"Slab - Eliminate lock_cpu_hotplug from slab"
(http://lkml.org/lkml/2006/10/28/92) by Ravikiran G Thirumalai and
Andrew Morton.

This patch adds two additional parameters to notifier_call_chain API namely
 - int nr_to_calls : Number of notifier_functions to be called.
 		     The don't care value is -1.

 - unsigned int *nr_calls : Records the total number of notifier_funtions
			    called by notifier_call_chain. The don't care
			    value is NULL.

[michal.k.k.piotrowski@gmail.com: build fix]
Credit: Andrew Morton <akpm@osdl.org>
Signed-off-by: Gautham R Shenoy <ego@in.ibm.com>
Signed-off-by: Michal Piotrowski <michal.k.k.piotrowski@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 30 insertions, 22 deletions

diff --git a/include/linux/notifier.h b/include/linux/notifier.h
index 10a43ed0527e..e34221bf8946 100644
--- a/include/linux/notifier.h
+++ b/include/linux/notifier.h
@@ -112,32 +112,40 @@ extern void srcu_init_notifier_head(struct srcu_notifier_head *nh);
 
 #ifdef __KERNEL__
 
-extern int atomic_notifier_chain_register(struct atomic_notifier_head *,
-		struct notifier_block *);
-extern int blocking_notifier_chain_register(struct blocking_notifier_head *,
-		struct notifier_block *);
-extern int raw_notifier_chain_register(struct raw_notifier_head *,
-		struct notifier_block *);
-extern int srcu_notifier_chain_register(struct srcu_notifier_head *,
-		struct notifier_block *);
-
-extern int atomic_notifier_chain_unregister(struct atomic_notifier_head *,
-		struct notifier_block *);
-extern int blocking_notifier_chain_unregister(struct blocking_notifier_head *,
-		struct notifier_block *);
-extern int raw_notifier_chain_unregister(struct raw_notifier_head *,
-		struct notifier_block *);
-extern int srcu_notifier_chain_unregister(struct srcu_notifier_head *,
-		struct notifier_block *);
-
-extern int atomic_notifier_call_chain(struct atomic_notifier_head *,
+extern int atomic_notifier_chain_register(struct atomic_notifier_head *nh,
+		struct notifier_block *nb);
+extern int blocking_notifier_chain_register(struct blocking_notifier_head *nh,
+		struct notifier_block *nb);
+extern int raw_notifier_chain_register(struct raw_notifier_head *nh,
+		struct notifier_block *nb);
+extern int srcu_notifier_chain_register(struct srcu_notifier_head *nh,
+		struct notifier_block *nb);
+
+extern int atomic_notifier_chain_unregister(struct atomic_notifier_head *nh,
+		struct notifier_block *nb);
+extern int blocking_notifier_chain_unregister(struct blocking_notifier_head *nh,
+		struct notifier_block *nb);
+extern int raw_notifier_chain_unregister(struct raw_notifier_head *nh,
+		struct notifier_block *nb);
+extern int srcu_notifier_chain_unregister(struct srcu_notifier_head *nh,
+		struct notifier_block *nb);
+
+extern int atomic_notifier_call_chain(struct atomic_notifier_head *nh,
 		unsigned long val, void *v);
-extern int blocking_notifier_call_chain(struct blocking_notifier_head *,
+extern int __atomic_notifier_call_chain(struct atomic_notifier_head *nh,
+	unsigned long val, void *v, int nr_to_call, int *nr_calls);
+extern int blocking_notifier_call_chain(struct blocking_notifier_head *nh,
 		unsigned long val, void *v);
-extern int raw_notifier_call_chain(struct raw_notifier_head *,
+extern int __blocking_notifier_call_chain(struct blocking_notifier_head *nh,
+	unsigned long val, void *v, int nr_to_call, int *nr_calls);
+extern int raw_notifier_call_chain(struct raw_notifier_head *nh,
 		unsigned long val, void *v);
-extern int srcu_notifier_call_chain(struct srcu_notifier_head *,
+extern int __raw_notifier_call_chain(struct raw_notifier_head *nh,
+	unsigned long val, void *v, int nr_to_call, int *nr_calls);
+extern int srcu_notifier_call_chain(struct srcu_notifier_head *nh,
 		unsigned long val, void *v);
+extern int __srcu_notifier_call_chain(struct srcu_notifier_head *nh,
+	unsigned long val, void *v, int nr_to_call, int *nr_calls);
 
 #define NOTIFY_DONE		0x0000		/* Don't care */
 #define NOTIFY_OK		0x0001		/* Suits me */