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>

diff --git a/fs/jbd2/checkpoint.c b/fs/jbd2/checkpoint.c
index 26f8d7e..02e0b79 100644 (file)
--- 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)

diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index 8ef6b6d..88d8f22 100644 (file)
--- 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);
        }