[PATCH] Fix try_to_free_buffer() locking

Fix commit ecdfc9787fe527491baefc22dce8b2dbd5b2908d

Not to put too fine a point on it, but in a nutshell...

	__set_page_dirty_buffers() | try_to_free_buffers()
	---------------------------+---------------------------
	                           | spin_lock(private_lock);
	                           | drop_bufers()
	                           | spin_unlock(private_lock);
	spin_lock(private_lock)    |
	!page_has_buffers()        |
	spin_unlock(private_lock)  |
	SetPageDirty()             |
	                           | cancel_dirty_page()

                          oops!

Signed-off-by: Nick Piggin <npiggin@suse.de>
Acked-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 1 deletions

diff --git a/fs/buffer.c b/fs/buffer.c
index 460f1c43238..1ad674fd348 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -2844,7 +2844,6 @@ int try_to_free_buffers(struct page *page)
 
         spin_lock(&mapping->private_lock);
         ret = drop_buffers(page, &buffers_to_free);
-        spin_unlock(&mapping->private_lock);
 
         /*
          * If the filesystem writes its buffers by hand (eg ext3)
@@ -2855,9 +2854,14 @@ int try_to_free_buffers(struct page *page)
          * Also, during truncate, discard_buffer will have marked all
          * the page's buffers clean.  We discover that here and clean
          * the page also.
+         *
+         * private_lock must be held over this entire operation in order
+         * to synchronise against __set_page_dirty_buffers and prevent the
+         * dirty bit from being lost.
          */
         if (ret)
                 cancel_dirty_page(page, PAGE_CACHE_SIZE);
+        spin_unlock(&mapping->private_lock);
 out:
         if (buffers_to_free) {
                 struct buffer_head *bh = buffers_to_free;