mmu_notifier: add call_srcu and sync function for listener to delay call and sync

When kernel device drivers or subsystems want to bind their lifespan to
t= he lifespan of the mm_struct, they usually use one of the following
methods:

1. Manually calling a function in the interested kernel module.  The
   funct= ion call needs to be placed in mmput.  This method was rejected
   by several ker= nel maintainers.

2. Registering to the mmu notifier release mechanism.

The problem with the latter approach is that the mmu_notifier_release
cal= lback is called from__mmu_notifier_release (called from exit_mmap).
That functi= on iterates over the list of mmu notifiers and don't expect
the release call= back function to remove itself from the list.
Therefore, the callback function= in the kernel module can't release the
mmu_notifier_object, which is actuall= y the kernel module's object
itself.  As a result, the destruction of the kernel module's object must
to be done in a delayed fashion.

This patch adds support for this delayed callback, by adding a new
mmu_notifier_call_srcu function that receives a function ptr and calls
th= at function with call_srcu.  In that function, the kernel module
releases its object.  To use mmu_notifier_call_srcu, the calling module
needs to call b= efore that a new function called
mmu_notifier_unregister_no_release that as its= name implies,
unregisters a notifier without calling its notifier release call= back.

This patch also adds a function that will call barrier_srcu so those
kern= el modules can sync with mmu_notifier.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Oded Gabbay <oded.gabbay@amd.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
index deca87452528b2888823a4daf4cb148d43a52544..27288692241eebe928d9ec6020dc3c42d13f3755 100644 (file)
--- 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 41cefdf0aaddc46144187cba72ddf3e2629a9f02..950813b1eb3656dc49e66eab4e912fff76dbfc2b 100644 (file)
--- a/mm/mmu_notifier.c
+++ b/mm/mmu_notifier.c
@@ -22,6 +22,25 @@
 /* global SRCU for all MMs */
 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
@@ -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);