[PATCH] ext3 jbd needs to wait for locked buffers

[PATCH] ext3 jbd needs to wait for locked buffers

[Chris Mason]

jbd needs to wait for any io to complete on the buffer before
changing the end_io function.  Using set_buffer_locked means that
it can change the end_io function while the page is in the middle of
writeback, and the writeback bit on the page will never get cleared.

Since we set the buffer dirty earlier on, if the page was
previously dirty, pdflush or memory pressure might trigger a
writepage call, which will race with jbd's set_buffer_locked.

Index: linux.t/fs/jbd/commit.c
===================================================================
--- linux.t.orig/fs/jbd/commit.c	2004-06-17 21:01:28.000000000 -0400
+++ linux.t/fs/jbd/commit.c	2004-06-17 21:02:16.000000000 -0400
@@ -504,7 +504,7 @@ write_out_data:
 start_journal_io:
 			for (i = 0; i < bufs; i++) {
 				struct buffer_head *bh = wbuf[i];
-				set_buffer_locked(bh);
+				lock_buffer(bh);
 				clear_buffer_dirty(bh);
 				set_buffer_uptodate(bh);
 				bh->b_end_io = journal_end_buffer_io_sync;

Re: [PATCH] ext3 jbd needs to wait for locked buffers

[Andrew Morton]

Chris Mason <mason@suse.com> wrote:
>
> jbd needs to wait for any io to complete on the buffer before
>  changing the end_io function.

Thanks, I bet that was fun to hunt down.

Same problem in 2.4, although there it looks like it'll just cause a memory
leak.  I guess if an app is mmapping a blockdev we could end up with a
permanently locked page though.