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Subject[PATCH 19/21] mm: dup_mmap down new mmap_sem
One anomaly remains from when Andrea rationalized the responsibilities
of mmap_sem and page_table_lock: in dup_mmap we add vmas to the child
holding its page_table_lock, but not the mmap_sem which normally guards
the vma list and rbtree. Which could be an issue for unuse_mm: though
since it just walks down the list (today with page_table_lock, tomorrow
not), it's probably okay. Will need a memory barrier? Oh, keep it
simple, Nick and I agreed, no harm in taking child's mmap_sem here.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
---
kernel/fork.c | 9 ++++-----
1 files changed, 4 insertions(+), 5 deletions(-)
--- mm18/kernel/fork.c 2005-09-24 19:30:21.000000000 +0100
+++ mm19/kernel/fork.c 2005-09-24 19:30:35.000000000 +0100
@@ -192,6 +192,8 @@ static inline int dup_mmap(struct mm_str

down_write(&oldmm->mmap_sem);
flush_cache_mm(oldmm);
+ down_write(&mm->mmap_sem);
+
mm->locked_vm = 0;
mm->mmap = NULL;
mm->mmap_cache = NULL;
@@ -251,10 +253,7 @@ static inline int dup_mmap(struct mm_str
}

/*
- * Link in the new vma and copy the page table entries:
- * link in first so that swapoff can see swap entries.
- * Note that, exceptionally, here the vma is inserted
- * without holding mm->mmap_sem.
+ * Link in the new vma and copy the page table entries.
*/
spin_lock(&mm->page_table_lock);
*pprev = tmp;
@@ -275,8 +274,8 @@ static inline int dup_mmap(struct mm_str
goto out;
}
retval = 0;
-
out:
+ up_write(&mm->mmap_sem);
flush_tlb_mm(oldmm);
up_write(&oldmm->mmap_sem);
return retval;
-
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