EMM: disable other notifiers before register and unregister

[Christoph Lameter]

Ok lets forget about the single theaded thing to solve the registration 
races. As Andrea pointed out this still has ssues with other subscribed 
subsystems (and also try_to_unmap). We could do something like what 
stop_machine_run does: First disable all running subsystems before 
registering a new one.

Maybe this is a possible solution.


Subject: EMM: disable other notifiers before register and unregister

As Andrea has pointed out: There are races during registration if other
subsystem notifiers are active while we register a callback.

Solve that issue by adding two new notifiers:

emm_stop
	Stops the notifier operations. Notifier must block on
	invalidate_start and emm_referenced from this point on.
	If an invalidate_start has not been completed by a call
	to invalidate_end then the driver must wait until the
	operation is complete before returning.

emm_start
	Restart notifier operations.

Before registration all other subscribed subsystems are stopped.
Then the new subsystem is subscribed and things can get running
without consistency issues.

Subsystems are restarted after the lists have been updated.

This also works for unregistering. If we can get all subsystems
to stop then we can also reliably unregister a subsystem. So
provide that callback.

Signed-off-by: Christoph Lameter <clameter@xxxxxxx>

---
 include/linux/rmap.h |   10 +++++++---
 mm/rmap.c            |   30 ++++++++++++++++++++++++++++++
 2 files changed, 37 insertions(+), 3 deletions(-)

Index: linux-2.6/include/linux/rmap.h
===================================================================
--- linux-2.6.orig/include/linux/rmap.h	2008-04-02 18:16:07.906032549 -0700
+++ linux-2.6/include/linux/rmap.h	2008-04-02 18:17:10.291070009 -0700
@@ -94,7 +94,9 @@ enum emm_operation {
 	emm_release,		/* Process exiting, */
 	emm_invalidate_start,	/* Before the VM unmaps pages */
 	emm_invalidate_end,	/* After the VM unmapped pages */
- 	emm_referenced		/* Check if a range was referenced */
+ 	emm_referenced,		/* Check if a range was referenced */
+	emm_stop,		/* Halt all faults/invalidate_starts */
+	emm_start,		/* Restart operations */
 };
 
 struct emm_notifier {
@@ -126,13 +128,15 @@ static inline int emm_notify(struct mm_s
 
 /*
  * Register a notifier with an mm struct. Release occurs when the process
- * terminates by calling the notifier function with emm_release.
+ * terminates by calling the notifier function with emm_release or when
+ * emm_notifier_unregister is called.
  *
  * Must hold the mmap_sem for write.
  */
 extern void emm_notifier_register(struct emm_notifier *e,
 					struct mm_struct *mm);
-
+extern void emm_notifier_unregister(struct emm_notifier *e,
+					struct mm_struct *mm);
 
 /*
  * Called from mm/vmscan.c to handle paging out
Index: linux-2.6/mm/rmap.c
===================================================================
--- linux-2.6.orig/mm/rmap.c	2008-04-02 18:16:09.378057062 -0700
+++ linux-2.6/mm/rmap.c	2008-04-02 18:16:10.710079201 -0700
@@ -289,16 +289,46 @@ void emm_notifier_release(struct mm_stru
 /* Register a notifier */
 void emm_notifier_register(struct emm_notifier *e, struct mm_struct *mm)
 {
+	/* Bring all other notifiers into a quiescent state */
+	emm_notify(mm, emm_stop, 0, TASK_SIZE);
+
 	e->next = mm->emm_notifier;
+
 	/*
 	 * The update to emm_notifier (e->next) must be visible
 	 * before the pointer becomes visible.
 	 * rcu_assign_pointer() does exactly what we need.
 	 */
 	rcu_assign_pointer(mm->emm_notifier, e);
+
+	/* Continue notifiers */
+	emm_notify(mm, emm_start, 0, TASK_SIZE);
 }
 EXPORT_SYMBOL_GPL(emm_notifier_register);
 
+/* Unregister a notifier */
+void emm_notifier_unregister(struct emm_notifier *e, struct mm_struct *mm)
+{
+	struct emm_notifier *p;
+
+	emm_notify(mm, emm_stop, 0, TASK_SIZE);
+
+	p = mm->emm_notifier;
+	if (e == p)
+		mm->emm_notifier = e->next;
+	else {
+		while (p->next != e)
+			p = p->next;
+
+		p->next = e->next;
+	}
+	e->next = mm->emm_notifier;
+
+	emm_notify(mm, emm_start, 0, TASK_SIZE);
+	e->callback(e, mm, emm_release, 0, TASK_SIZE);
+}
+EXPORT_SYMBOL_GPL(emm_notifier_unregister);
+
 /*
  * Perform a callback
  *

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