linux-ext4 - [patch 32/35] jbd: Fix assertion failure in fs/jbd/checkpoint.c

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Message-Id: <200712050745.lB57jRTr027560@imap1.linux-foundation.org>
Date:	Tue, 04 Dec 2007 23:45:27 -0800
From:	akpm@...ux-foundation.org
To:	torvalds@...ux-foundation.org
Cc:	akpm@...ux-foundation.org, jack@...e.cz, linux-ext4@...r.kernel.org
Subject: [patch 32/35] jbd: Fix assertion failure in fs/jbd/checkpoint.c

From: Jan Kara <jack@...e.cz>

Before we start committing a transaction, we call
__journal_clean_checkpoint_list() to cleanup transaction's written-back
buffers.

If this call happens to remove all of them (and there were already some
buffers), __journal_remove_checkpoint() will decide to free the transaction
because it isn't (yet) a committing transaction and soon we fail some
assertion - the transaction really isn't ready to be freed :).

We change the check in __journal_remove_checkpoint() to free only a
transaction in T_FINISHED state.  The locking there is subtle though (as
everywhere in JBD ;().  We use j_list_lock to protect the check and a
subsequent call to __journal_drop_transaction() and do the same in the end
of journal_commit_transaction() which is the only place where a transaction
can get to T_FINISHED state.

Probably I'm too paranoid here and such locking is not really necessary -
checkpoint lists are processed only from log_do_checkpoint() where a
transaction must be already committed to be processed or from
__journal_clean_checkpoint_list() where kjournald itself calls it and thus
transaction cannot change state either.  Better be safe if something
changes in future...

Signed-off-by: Jan Kara <jack@...e.cz>
Cc: <linux-ext4@...r.kernel.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
---

 fs/jbd/checkpoint.c |   12 ++++++------
 fs/jbd/commit.c     |    8 ++++----
 include/linux/jbd.h |    2 ++
 3 files changed, 12 insertions(+), 10 deletions(-)

diff -puN fs/jbd/checkpoint.c~jbd-fix-assertion-failure-in-fs-jbd-checkpointc fs/jbd/checkpoint.c
--- a/fs/jbd/checkpoint.c~jbd-fix-assertion-failure-in-fs-jbd-checkpointc
+++ a/fs/jbd/checkpoint.c
@@ -602,15 +602,15 @@ int __journal_remove_checkpoint(struct j
 
 	/*
 	 * There is one special case to worry about: if we have just pulled the
-	 * buffer off a committing transaction's forget list, then even if the
-	 * checkpoint list is empty, the transaction obviously cannot be
-	 * dropped!
+	 * buffer off a running or committing transaction's checkpoing list,
+	 * then even if the checkpoint list is empty, the transaction obviously
+	 * cannot be dropped!
 	 *
-	 * The locking here around j_committing_transaction is a bit sleazy.
+	 * The locking here around t_state is a bit sleazy.
 	 * See the comment at the end of journal_commit_transaction().
 	 */
-	if (transaction == journal->j_committing_transaction) {
-		JBUFFER_TRACE(jh, "belongs to committing transaction");
+	if (transaction->t_state != T_FINISHED) {
+		JBUFFER_TRACE(jh, "belongs to running/committing transaction");
 		goto out;
 	}
 
diff -puN fs/jbd/commit.c~jbd-fix-assertion-failure-in-fs-jbd-checkpointc fs/jbd/commit.c
--- a/fs/jbd/commit.c~jbd-fix-assertion-failure-in-fs-jbd-checkpointc
+++ a/fs/jbd/commit.c
@@ -858,10 +858,10 @@ restart_loop:
 	}
 	spin_unlock(&journal->j_list_lock);
 	/*
-	 * This is a bit sleazy.  We borrow j_list_lock to protect
-	 * journal->j_committing_transaction in __journal_remove_checkpoint.
-	 * Really, __journal_remove_checkpoint should be using j_state_lock but
-	 * it's a bit hassle to hold that across __journal_remove_checkpoint
+	 * This is a bit sleazy.  We use j_list_lock to protect transition
+	 * of a transaction into T_FINISHED state and calling
+	 * __journal_drop_transaction(). Otherwise we could race with
+	 * other checkpointing code processing the transaction...
 	 */
 	spin_lock(&journal->j_state_lock);
 	spin_lock(&journal->j_list_lock);
diff -puN include/linux/jbd.h~jbd-fix-assertion-failure-in-fs-jbd-checkpointc include/linux/jbd.h
--- a/include/linux/jbd.h~jbd-fix-assertion-failure-in-fs-jbd-checkpointc
+++ a/include/linux/jbd.h
@@ -439,6 +439,8 @@ struct transaction_s
 	/*
 	 * Transaction's current state
 	 * [no locking - only kjournald alters this]
+	 * [j_list_lock] guards transition of a transaction into T_FINISHED
+	 * state and subsequent call of __journal_drop_transaction()
 	 * FIXME: needs barriers
 	 * KLUDGE: [use j_state_lock]
 	 */
_
-
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