[PATCH] ext4: fix deadlock while checkpoint thread waits commit thread to finish

[Xiaoguang Wang <xiaoguang.wang@xxxxxxxxxxxxxxxxx>]

This issue was found when I tried to put checkpoint work in a separate thread,
the deadlock below happened:
         Thread1                                |   Thread2
__jbd2_log_wait_for_space                       |
jbd2_log_do_checkpoint (hold j_checkpoint_mutex)|
  if (jh->b_transaction != NULL)                |
    ...                                         |
    jbd2_log_start_commit(journal, tid);        |jbd2_update_log_tail
                                                |  will lock j_checkpoint_mutex,
                                                |  but will be blocked here.
                                                |
    jbd2_log_wait_commit(journal, tid);         |
    wait_event(journal->j_wait_done_commit,     |
     !tid_gt(tid, journal->j_commit_sequence)); |
     ...                                        |wake_up(j_wait_done_commit)
  }                                             |

then deadlock occurs, Thread1 will never be waken up.

To fix this issue, drop j_checkpoint_mutex in jbd2_log_do_checkpoint()
when we are going to wait for transaction commit.

Signed-off-by: Xiaoguang Wang <xiaoguang.wang@xxxxxxxxxxxxxxxxx>
---
 fs/jbd2/checkpoint.c | 21 +++++++++++++++++----
 fs/jbd2/journal.c    |  2 +-
 2 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/fs/jbd2/checkpoint.c b/fs/jbd2/checkpoint.c
index 26f8d7e46462..e728844f2f0e 100644
--- a/fs/jbd2/checkpoint.c
+++ b/fs/jbd2/checkpoint.c
@@ -113,7 +113,7 @@ void __jbd2_log_wait_for_space(journal_t *journal)
 	nblocks = jbd2_space_needed(journal);
 	while (jbd2_log_space_left(journal) < nblocks) {
 		write_unlock(&journal->j_state_lock);
-		mutex_lock(&journal->j_checkpoint_mutex);
+		mutex_lock_io(&journal->j_checkpoint_mutex);
 
 		/*
 		 * Test again, another process may have checkpointed while we
@@ -241,8 +241,8 @@ int jbd2_log_do_checkpoint(journal_t *journal)
 	 * done (maybe it's a new transaction, but it fell at the same
 	 * address).
 	 */
-	if (journal->j_checkpoint_transactions != transaction ||
-	    transaction->t_tid != this_tid)
+	if (journal->j_checkpoint_transactions == NULL ||
+	    journal->j_checkpoint_transactions->t_tid != this_tid)
 		goto out;
 
 	/* checkpoint all of the transaction's buffers */
@@ -276,9 +276,22 @@ int jbd2_log_do_checkpoint(journal_t *journal)
 		"JBD2: %s: Waiting for Godot: block %llu\n",
 		journal->j_devname, (unsigned long long) bh->b_blocknr);
 
+			if (batch_count)
+				__flush_batch(journal, &batch_count);
 			jbd2_log_start_commit(journal, tid);
+			/*
+			 * jbd2_journal_commit_transaction() may want
+			 * to take the checkpoint_mutex if JBD2_FLUSHED
+			 * is set, jbd2_update_log_tail() called by
+			 * jbd2_journal_commit_transaction() may also take
+			 * checkpoint_mutex.  So we need to temporarily
+			 * drop it.
+			 */
+			mutex_unlock(&journal->j_checkpoint_mutex);
 			jbd2_log_wait_commit(journal, tid);
-			goto retry;
+			mutex_lock_io(&journal->j_checkpoint_mutex);
+			spin_lock(&journal->j_list_lock);
+			goto restart;
 		}
 		if (!buffer_dirty(bh)) {
 			if (unlikely(buffer_write_io_error(bh)) && !result)
diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index 8ef6b6daaa7a..88d8f22d2cba 100644
--- a/fs/jbd2/journal.c
+++ b/fs/jbd2/journal.c
@@ -2067,7 +2067,7 @@ int jbd2_journal_wipe(journal_t *journal, int write)
 	err = jbd2_journal_skip_recovery(journal);
 	if (write) {
 		/* Lock to make assertions happy... */
-		mutex_lock(&journal->j_checkpoint_mutex);
+		mutex_lock_io(&journal->j_checkpoint_mutex);
 		jbd2_mark_journal_empty(journal, REQ_SYNC | REQ_FUA);
 		mutex_unlock(&journal->j_checkpoint_mutex);
 	}
-- 
2.17.2