jbd2: fix deadlock while checkpoint thread waits commit thread to finish

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.

Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

1 files changed, 15 insertions, 2 deletions

diff --git a/fs/jbd2/checkpoint.c b/fs/jbd2/checkpoint.c
index 26f8d7e46462..02e0b79753e7 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
@@ -276,9 +276,22 @@ restart:
                 "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)