linux-kernel - Re: 2.6.18 ext3 panic.

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Date:	Thu, 12 Oct 2006 15:07:11 -0500
From:	Eric Sandeen <esandeen@...hat.com>
To:	Andrew Morton <akpm@...l.org>
CC:	Jan Kara <jack@...e.cz>, Badari Pulavarty <pbadari@...ibm.com>,
	Eric Sandeen <sandeen@...deen.net>,
	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: 2.6.18 ext3 panic.

Andrew Morton wrote:

> On Thu, 12 Oct 2006 14:28:20 +0200
> Jan Kara <jack@...e.cz> wrote:
>
>   
>> Where can we call
>> journal_dirty_data() without PageLock?
>>     
>
> block_write_full_page() will unlock the page, so ext3_writepage()
> will run journal_dirty_data_fn() against an unlocked page.
>
> I haven't looked into the exact details of the race, but it should
> be addressable via jbd_lock_bh_state() or j_list_lock coverage
I'm testing with something like this now; seem sane?

journal_dirty_data & journal_unmap_data both check do 
jbd_lock_bh_state(bh) close to the top... journal_dirty_data_fn has checked 
buffer_mapped before getting into journal_dirty_data, but that state may
change before the lock is grabbed.  Similarly re-check after we drop the lock.

-Eric

Index: linux-2.6.18-1.2737.fc6/fs/jbd/transaction.c
===================================================================
--- linux-2.6.18-1.2737.fc6.orig/fs/jbd/transaction.c
+++ linux-2.6.18-1.2737.fc6/fs/jbd/transaction.c
@@ -967,6 +967,13 @@ int journal_dirty_data(handle_t *handle,
 	 */
 	jbd_lock_bh_state(bh);
 	spin_lock(&journal->j_list_lock);
+
+	/* Now that we have bh_state locked, are we really still mapped? */
+	if (!buffer_mapped(bh)) {
+		JBUFFER_TRACE(jh, "unmapped, bailing out");
+		goto no_journal;
+	}
+		
 	if (jh->b_transaction) {
 		JBUFFER_TRACE(jh, "has transaction");
 		if (jh->b_transaction != handle->h_transaction) {
@@ -1028,6 +1036,11 @@ int journal_dirty_data(handle_t *handle,
 				sync_dirty_buffer(bh);
 				jbd_lock_bh_state(bh);
 				spin_lock(&journal->j_list_lock);
+				/* Since we dropped the lock... */
+				if (!buffer_mapped(bh)) {
+					JBUFFER_TRACE(jh, "Got unmapped");
+					goto no_journal;
+				}
 				/* The buffer may become locked again at any
 				   time if it is redirtied */
 			}

-
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