How about just taking the mmap_sem writelock in release? We have only a
single caller of mmu_notifier_release() in mm/mmap.c and we know that we
are not holding mmap_sem at that point. So just acquire it when needed?
Index: linux-2.6/mm/mmu_notifier.c
===================================================================
--- linux-2.6.orig/mm/mmu_notifier.c 2008-01-30 11:21:57.000000000 -0800
+++ linux-2.6/mm/mmu_notifier.c 2008-01-30 11:24:59.000000000 -0800
@@ -18,6 +19,7 @@ void mmu_notifier_release(struct mm_stru
struct hlist_node *n, *t;
if (unlikely(!hlist_empty(&mm->mmu_notifier.head))) {
+ down_write(&mm->mmap_sem);
rcu_read_lock();
hlist_for_each_entry_safe_rcu(mn, n, t,
&mm->mmu_notifier.head, hlist) {
@@ -26,6 +28,7 @@ void mmu_notifier_release(struct mm_stru
mn->ops->release(mn, mm);
}
rcu_read_unlock();
+ up_write(&mm->mmap_sem);
synchronize_rcu();
}
}
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