linux-ext4 - [PATCH 1/4] jbd2/journal_commit_transaction: relocate state lock to incorporate all users

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Date:	Mon, 10 Jun 2013 15:32:00 -0400
From:	Paul Gortmaker <paul.gortmaker@...driver.com>
To:	<linux-ext4@...r.kernel.org>
CC:	<linux-rt-users@...r.kernel.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>
Subject: [PATCH 1/4] jbd2/journal_commit_transaction: relocate state lock to incorporate all users

The state lock is taken after we are doing an assert on the state
value, not before.  It is also taken after the jbd_debug() call.
Noting that jbd_debug() is implemented with two separate printk()
calls, it can lead to corrupted and misleading debug output like
the following (see lines marked with "*"):

[  290.339362] (fs/jbd2/journal.c, 203): kjournald2: kjournald2 wakes
[  290.339365] (fs/jbd2/journal.c, 155): kjournald2: commit_sequence=42103, commit_request=42104
[  290.339369] (fs/jbd2/journal.c, 158): kjournald2: OK, requests differ
[* 290.339376] (fs/jbd2/journal.c, 648): jbd2_log_wait_commit:
[* 290.339379] (fs/jbd2/commit.c, 370): jbd2_journal_commit_transaction: JBD2: want 42104, j_commit_sequence=42103
[* 290.339382] JBD2: starting commit of transaction 42104
[  290.339410] (fs/jbd2/revoke.c, 566): jbd2_journal_write_revoke_records: Wrote 0 revoke records
[  290.376555] (fs/jbd2/commit.c, 1088): jbd2_journal_commit_transaction: JBD2: commit 42104 complete, head 42079

i.e. the debug output from log_wait_commit and journal_commit_transaction
have become interleaved.  The output should have been:

(fs/jbd2/journal.c, 648): jbd2_log_wait_commit: JBD2: want 42104, j_commit_sequence=42103
(fs/jbd2/commit.c, 370): jbd2_journal_commit_transaction: JBD2: starting commit of transaction 42104

It is expected that this is not easy to replicate -- I was only able
to cause it on preempt-rt kernels, and even then only under heavy
I/O load.

Signed-off-by: Paul Gortmaker <paul.gortmaker@...driver.com>
---
 fs/jbd2/commit.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
index 0f53946..7e8a5fd 100644
--- a/fs/jbd2/commit.c
+++ b/fs/jbd2/commit.c
@@ -423,6 +423,8 @@ void jbd2_journal_commit_transaction(journal_t *journal)
 	J_ASSERT(journal->j_running_transaction != NULL);
 	J_ASSERT(journal->j_committing_transaction == NULL);
 
+	write_lock(&journal->j_state_lock);
+
 	commit_transaction = journal->j_running_transaction;
 	J_ASSERT(commit_transaction->t_state == T_RUNNING);
 
@@ -430,7 +432,6 @@ void jbd2_journal_commit_transaction(journal_t *journal)
 	jbd_debug(1, "JBD2: starting commit of transaction %d\n",
 			commit_transaction->t_tid);
 
-	write_lock(&journal->j_state_lock);
 	commit_transaction->t_state = T_LOCKED;
 
 	trace_jbd2_commit_locking(journal, commit_transaction);
-- 
1.8.1.2

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