Commit d76a3a77 authored by Theodore Ts'o's avatar Theodore Ts'o
Browse files

ext4/jbd2: don't wait (forever) for stale tid caused by wraparound 



In the case where an inode has a very stale transaction id (tid) in
i_datasync_tid or i_sync_tid, it's possible that after a very large
(2**31) number of transactions, that the tid number space might wrap,
causing tid_geq()'s calculations to fail.

Commit deeeaf13 "jbd2: fix fsync() tid wraparound bug", later modified
by commit e7b04ac0 "jbd2: don't wake kjournald unnecessarily",
attempted to fix this problem, but it only avoided kjournald spinning
forever by fixing the logic in jbd2_log_start_commit().

Unfortunately, in the codepaths in fs/ext4/fsync.c and fs/ext4/inode.c
that might call jbd2_log_start_commit() with a stale tid, those
functions will subsequently call jbd2_log_wait_commit() with the same
stale tid, and then wait for a very long time.  To fix this, we
replace the calls to jbd2_log_start_commit() and
jbd2_log_wait_commit() with a call to a new function,
jbd2_complete_transaction(), which will correctly handle stale tid's.

As a bonus, jbd2_complete_transaction() will avoid locking
j_state_lock for writing unless a commit needs to be started.  This
should have a small (but probably not measurable) improvement for
ext4's scalability.

Signed-off-by: default avatar"Theodore Ts'o" <tytso@mit.edu>
Reported-by: default avatarBen Hutchings <ben@decadent.org.uk>
Reported-by: default avatarGeorge Barnett <gbarnett@atlassian.com>
Cc: stable@vger.kernel.org
parent b10a44c3

fs/ext4/fsync.c

+1 −2
Original line number Diff line number Diff line
@@ -166,8 +166,7 @@ int ext4_sync_file(struct file *file, loff_t start, loff_t end, int datasync) 
	if (journal->j_flags & JBD2_BARRIER &&

	    !jbd2_trans_will_send_data_barrier(journal, commit_tid))

		needs_barrier = true;

	jbd2_log_start_commit(journal, commit_tid);

	ret = jbd2_log_wait_commit(journal, commit_tid);

	ret = jbd2_complete_transaction(journal, commit_tid);

	if (needs_barrier) {

		err = blkdev_issue_flush(inode->i_sb->s_bdev, GFP_KERNEL, NULL);

		if (!ret)

fs/ext4/inode.c

+1 −2
Original line number Diff line number Diff line
@@ -210,8 +210,7 @@ void ext4_evict_inode(struct inode *inode) 
			journal_t *journal = EXT4_SB(inode->i_sb)->s_journal;

			tid_t commit_tid = EXT4_I(inode)->i_datasync_tid;



			jbd2_log_start_commit(journal, commit_tid);

			jbd2_log_wait_commit(journal, commit_tid);

			jbd2_complete_transaction(journal, commit_tid);

			filemap_write_and_wait(&inode->i_data);

		}

		truncate_inode_pages(&inode->i_data, 0);

fs/jbd2/journal.c

+31 −0
Original line number Diff line number Diff line
@@ -709,6 +709,37 @@ int jbd2_log_wait_commit(journal_t *journal, tid_t tid) 
	return err;

}



/*

 * When this function returns the transaction corresponding to tid

 * will be completed.  If the transaction has currently running, start

 * committing that transaction before waiting for it to complete.  If

 * the transaction id is stale, it is by definition already completed,

 * so just return SUCCESS.

 */

int jbd2_complete_transaction(journal_t *journal, tid_t tid)

{

	int	need_to_wait = 1;



	read_lock(&journal->j_state_lock);

	if (journal->j_running_transaction &&

	    journal->j_running_transaction->t_tid == tid) {

		if (journal->j_commit_request != tid) {

			/* transaction not yet started, so request it */

			read_unlock(&journal->j_state_lock);

			jbd2_log_start_commit(journal, tid);

			goto wait_commit;

		}

	} else if (!(journal->j_committing_transaction &&

		     journal->j_committing_transaction->t_tid == tid))

		need_to_wait = 0;

	read_unlock(&journal->j_state_lock);

	if (!need_to_wait)

		return 0;

wait_commit:

	return jbd2_log_wait_commit(journal, tid);

}

EXPORT_SYMBOL(jbd2_complete_transaction);



/*

 * Log buffer allocation routines:

 */

include/linux/jbd2.h

+1 −0
Original line number Diff line number Diff line
@@ -1200,6 +1200,7 @@ int __jbd2_log_start_commit(journal_t *journal, tid_t tid); 
int jbd2_journal_start_commit(journal_t *journal, tid_t *tid);

int jbd2_journal_force_commit_nested(journal_t *journal);

int jbd2_log_wait_commit(journal_t *journal, tid_t tid);

int jbd2_complete_transaction(journal_t *journal, tid_t tid);

int jbd2_log_do_checkpoint(journal_t *journal);

int jbd2_trans_will_send_data_barrier(journal_t *journal, tid_t tid);

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