[PATCH 19/21] mm: dup_mmap down new mmap_sem

[Hugh Dickins]

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@xxxxxxxxxxx>
---

 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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