[PATCH 1/7] mmu_notifier: add call_srcu and sync function for listener to delay call and sync.

[j.glisse@gmail.com]

From: Peter Zijlstra <peterz@infradead.org>

New mmu_notifier listener are eager to cleanup there structure after the
mmu_notifier::release callback. In order to allow this the patch provide
a function that allows to add a delayed call to the mmu_notifier srcu. It
also add a function that will call barrier_srcu so those listener can sync
with mmu_notifier.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: JA(C)rA'me Glisse <jglisse@redhat.com>
---
 include/linux/mmu_notifier.h |  6 ++++++
 mm/mmu_notifier.c            | 40 +++++++++++++++++++++++++++++++++++++++-
 2 files changed, 45 insertions(+), 1 deletion(-)

diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
index deca874..2728869 100644
--- a/include/linux/mmu_notifier.h
+++ b/include/linux/mmu_notifier.h
@@ -170,6 +170,8 @@ extern int __mmu_notifier_register(struct mmu_notifier *mn,
 				   struct mm_struct *mm);
 extern void mmu_notifier_unregister(struct mmu_notifier *mn,
 				    struct mm_struct *mm);
+extern void mmu_notifier_unregister_no_release(struct mmu_notifier *mn,
+					       struct mm_struct *mm);
 extern void __mmu_notifier_mm_destroy(struct mm_struct *mm);
 extern void __mmu_notifier_release(struct mm_struct *mm);
 extern int __mmu_notifier_clear_flush_young(struct mm_struct *mm,
@@ -288,6 +290,10 @@ static inline void mmu_notifier_mm_destroy(struct mm_struct *mm)
 	set_pte_at(___mm, ___address, __ptep, ___pte);			\
 })
 
+extern void mmu_notifier_call_srcu(struct rcu_head *rcu,
+				   void (*func)(struct rcu_head *rcu));
+extern void mmu_notifier_synchronize(void);
+
 #else /* CONFIG_MMU_NOTIFIER */
 
 static inline void mmu_notifier_release(struct mm_struct *mm)
diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
index 41cefdf..950813b 100644
--- a/mm/mmu_notifier.c
+++ b/mm/mmu_notifier.c
@@ -23,6 +23,25 @@
 static struct srcu_struct srcu;
 
 /*
+ * This function allows mmu_notifier::release callback to delay a call to
+ * a function that will free appropriate resources. The function must be
+ * quick and must not block.
+ */
+void mmu_notifier_call_srcu(struct rcu_head *rcu,
+			    void (*func)(struct rcu_head *rcu))
+{
+	call_srcu(&srcu, rcu, func);
+}
+EXPORT_SYMBOL_GPL(mmu_notifier_call_srcu);
+
+void mmu_notifier_synchronize(void)
+{
+	/* Wait for any running method to finish. */
+	srcu_barrier(&srcu);
+}
+EXPORT_SYMBOL_GPL(mmu_notifier_synchronize);
+
+/*
  * This function can't run concurrently against mmu_notifier_register
  * because mm->mm_users > 0 during mmu_notifier_register and exit_mmap
  * runs with mm_users == 0. Other tasks may still invoke mmu notifiers
@@ -53,7 +72,6 @@ void __mmu_notifier_release(struct mm_struct *mm)
 		 */
 		if (mn->ops->release)
 			mn->ops->release(mn, mm);
-	srcu_read_unlock(&srcu, id);
 
 	spin_lock(&mm->mmu_notifier_mm->lock);
 	while (unlikely(!hlist_empty(&mm->mmu_notifier_mm->list))) {
@@ -69,6 +87,7 @@ void __mmu_notifier_release(struct mm_struct *mm)
 		hlist_del_init_rcu(&mn->hlist);
 	}
 	spin_unlock(&mm->mmu_notifier_mm->lock);
+	srcu_read_unlock(&srcu, id);
 
 	/*
 	 * synchronize_srcu here prevents mmu_notifier_release from returning to
@@ -325,6 +344,25 @@ void mmu_notifier_unregister(struct mmu_notifier *mn, struct mm_struct *mm)
 }
 EXPORT_SYMBOL_GPL(mmu_notifier_unregister);
 
+/*
+ * Same as mmu_notifier_unregister but no callback and no srcu synchronization.
+ */
+void mmu_notifier_unregister_no_release(struct mmu_notifier *mn,
+					struct mm_struct *mm)
+{
+	spin_lock(&mm->mmu_notifier_mm->lock);
+	/*
+	 * Can not use list_del_rcu() since __mmu_notifier_release
+	 * can delete it before we hold the lock.
+	 */
+	hlist_del_init_rcu(&mn->hlist);
+	spin_unlock(&mm->mmu_notifier_mm->lock);
+
+	BUG_ON(atomic_read(&mm->mm_count) <= 0);
+	mmdrop(mm);
+}
+EXPORT_SYMBOL_GPL(mmu_notifier_unregister_no_release);
+
 static int __init mmu_notifier_init(void)
 {
 	return init_srcu_struct(&srcu);
-- 
1.9.0

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