Re: [PATCH] rmap: make anon_vma_prepare link in all the anon_vmasof a mergeable VMA

From: Rik van Riel
Date: Wed Apr 07 2010 - 10:49:58 EST

On 04/07/2010 03:00 AM, KOSAKI Motohiro wrote:

Hmm.. probably I'm moron.

Someone might be, but it's not you :)

I'm also confusing this locking rule as same as linus said.

after this patch, new locking order are

So, Why mmap_sem read lock can protect vma->anon_vma_chain?
An another threads seems to be able to change avc list concurrentlly and freely.

You are right, the code needs to take the pagetable_lock
around the call to anon_vma_clone, so other threads
get locked out.

This means the locking order has now been inverted,
with the pagetable_lock on the outside and the
anon_vma locks on the inside.

I have checked all the other call sites to the
anon_vma code. The direct callers of anon_vma_clone
and anon_vma_fork already hold the mmap_sem for
write. The callers of anon_vma_prepare hold the
mmap_sem for read - so excluding other callers of
anon_vma_prepare with the page_table_lock is enough.

mm_take_all_locks has the mmap_sem for write.

There seem to be no other traversals of the same_vma
list, so changing the locking order to have the
page_table_lock on the outside of the anon_vma locks

plus, Why don't we need "vma->anon_vma = merge_vma->anon_vma" assignment?
if vma->anon_vma keep NULL, I think anon_vma_prepare() call anon_vma_clone()
multiple times.

Added in the new version. See the next email.

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