[PATCH 2/3] jbd2: commit as soon as possible after log_start_commit

[Theodore Ts'o <tytso@mit.edu>]

Once a transaction has been requested to be committed, don't let any
other handles start under that transaction, and don't allow any
pending transactions to be extended (i.e., in the case of
unlink/ftruncate).

The idea is once the transaction has had log_start_commit() called on
it, at least one thread is blocked waiting for that transaction to
commit, and over time, more and more threads will end up getting
blocked.  In order to avoid high variability in file system operations
getting blocked behind the a blocked start_this_handle(), we should
try to get the commit started as soon as possible.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
---
 fs/jbd2/commit.c      | 3 ++-
 fs/jbd2/journal.c     | 1 +
 fs/jbd2/transaction.c | 6 ++++--
 include/linux/jbd2.h  | 1 +
 4 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
index 3091d42..fd2ac94 100644
--- a/fs/jbd2/commit.c
+++ b/fs/jbd2/commit.c
@@ -424,7 +424,8 @@ void jbd2_journal_commit_transaction(journal_t *journal)
 	J_ASSERT(journal->j_committing_transaction == NULL);
 
 	commit_transaction = journal->j_running_transaction;
-	J_ASSERT(commit_transaction->t_state == T_RUNNING);
+	J_ASSERT(commit_transaction->t_state == T_REQUESTED ||
+		 commit_transaction->t_state == T_RUNNING);
 
 	trace_jbd2_start_commit(journal, commit_transaction);
 	jbd_debug(1, "JBD2: starting commit of transaction %d\n",
diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index 1a80e31..c22773b 100644
--- a/fs/jbd2/journal.c
+++ b/fs/jbd2/journal.c
@@ -533,6 +533,7 @@ int __jbd2_log_start_commit(journal_t *journal, tid_t target)
 		jbd_debug(1, "JBD2: requesting commit %d/%d\n",
 			  journal->j_commit_request,
 			  journal->j_commit_sequence);
+		journal->j_running_transaction->t_state = T_REQUESTED;
 		wake_up(&journal->j_wait_commit);
 		return 1;
 	} else if (!tid_geq(journal->j_commit_request, target))
diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
index df9f297..6daff29 100644
--- a/fs/jbd2/transaction.c
+++ b/fs/jbd2/transaction.c
@@ -224,7 +224,8 @@ repeat:
 	 * If the current transaction is locked down for commit, wait for the
 	 * lock to be released.
 	 */
-	if (transaction->t_state == T_LOCKED) {
+	if ((transaction->t_state == T_LOCKED) ||
+	    (transaction->t_state == T_REQUESTED)) {
 		DEFINE_WAIT(wait);
 
 		prepare_to_wait(&journal->j_wait_transaction_locked,
@@ -2179,7 +2180,8 @@ void __jbd2_journal_refile_buffer(struct journal_head *jh)
 	else
 		jlist = BJ_Reserved;
 	__jbd2_journal_file_buffer(jh, jh->b_transaction, jlist);
-	J_ASSERT_JH(jh, jh->b_transaction->t_state == T_RUNNING);
+	J_ASSERT_JH(jh, (jh->b_transaction->t_state == T_RUNNING ||
+			 jh->b_transaction->t_state == T_REQUESTED));
 
 	if (was_dirty)
 		set_buffer_jbddirty(bh);
diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
index e30b663..920a8d0 100644
--- a/include/linux/jbd2.h
+++ b/include/linux/jbd2.h
@@ -493,6 +493,7 @@ struct transaction_s
 	 */
 	enum {
 		T_RUNNING,
+		T_REQUESTED,
 		T_LOCKED,
 		T_FLUSH,
 		T_COMMIT,
-- 
1.7.12.rc0.22.gcdd159b