[PATCH] Fix race in do_get_write_access()

  attached patch should fix the following race:
     Proc 1                               Proc 2

     __flush_batch()
       ll_rw_block()
                                        do_get_write_access()
					   lock_buffer
                                             jh is only waiting for checkpoint
					     -> b_transaction == NULL ->
					     do nothing
                                           unlock_buffer
    test_set_buffer_locked()
    test_clear_buffer_dirty()
                                           __journal_file_buffer()
                                        change the data
    submit_bh()

and we have sent wrong data to disk...  We now clean the dirty buffer flag
under buffer lock in all cases and hence we know that whenever a buffer is
starting to be journaled we either finish the pending write-out before
attaching a buffer to a transaction or we won't write the buffer until the
transaction is going to be committed.

The test in jbd_unexpected_dirty_buffer() is redundant - remove it.
Furthermore we have to clear the buffer dirty bit under the buffer lock to
prevent races with buffer write-out (and hence prevent returning a buffer with
IO happening).

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 21 insertions, 18 deletions

diff --git a/fs/jbd/transaction.c b/fs/jbd/transaction.c
index 77b7662b840b..c6ec66fd8766 100644
--- a/fs/jbd/transaction.c
+++ b/fs/jbd/transaction.c
@@ -490,23 +490,21 @@ void journal_unlock_updates (journal_t *journal)
  */
 static void jbd_unexpected_dirty_buffer(struct journal_head *jh)
 {
-        struct buffer_head *bh = jh2bh(jh);
         int jlist;
 
-        if (buffer_dirty(bh)) {
-                /* If this buffer is one which might reasonably be dirty
-                 * --- ie. data, or not part of this journal --- then
-                 * we're OK to leave it alone, but otherwise we need to
-                 * move the dirty bit to the journal's own internal
-                 * JBDDirty bit. */
-                jlist = jh->b_jlist;
-
-                if (jlist == BJ_Metadata || jlist == BJ_Reserved || 
-                    jlist == BJ_Shadow || jlist == BJ_Forget) {
-                        if (test_clear_buffer_dirty(jh2bh(jh))) {
-                                set_bit(BH_JBDDirty, &jh2bh(jh)->b_state);
-                        }
-                }
+        /* If this buffer is one which might reasonably be dirty
+         * --- ie. data, or not part of this journal --- then
+         * we're OK to leave it alone, but otherwise we need to
+         * move the dirty bit to the journal's own internal
+         * JBDDirty bit. */
+        jlist = jh->b_jlist;
+
+        if (jlist == BJ_Metadata || jlist == BJ_Reserved ||
+            jlist == BJ_Shadow || jlist == BJ_Forget) {
+                struct buffer_head *bh = jh2bh(jh);
+
+                if (test_clear_buffer_dirty(bh))
+                        set_buffer_jbddirty(bh);
         }
 }
 
@@ -574,9 +572,14 @@ repeat:
                         if (jh->b_next_transaction)
                                 J_ASSERT_JH(jh, jh->b_next_transaction ==
                                                         transaction);
-                        JBUFFER_TRACE(jh, "Unexpected dirty buffer");
-                        jbd_unexpected_dirty_buffer(jh);
-                }
+                }
+                /*
+                 * In any case we need to clean the dirty flag and we must
+                 * do it under the buffer lock to be sure we don't race
+                 * with running write-out.
+                 */
+                JBUFFER_TRACE(jh, "Unexpected dirty buffer");
+                jbd_unexpected_dirty_buffer(jh);
         }
 
         unlock_buffer(bh);