Commit 53cf9784 authored by Xiaoguang Wang's avatar Xiaoguang Wang Committed by Theodore Ts'o
Browse files

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: default avatarJan Kara <jack@suse.cz>
Signed-off-by: default avatarXiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Signed-off-by: default avatarTheodore Ts'o <tytso@mit.edu>
parent 8fdd60f2

fs/jbd2/checkpoint.c

+15 −2
Original line number Diff line number Diff line
@@ -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)

fs/jbd2/journal.c

+1 −1
Original line number Diff line number Diff line
@@ -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);

	}

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