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Commit be4f27d3 authored by Yongqiang Yang's avatar Yongqiang Yang Committed by Theodore Ts'o
Browse files

ext4: allow an active handle to be started when freezing 



ext4_journal_start_sb() should not prevent an active handle from being
started due to s_frozen.  Otherwise, deadlock is easy to happen, below
is a situation.

================================================
     freeze         |       truncate
================================================
                    |  ext4_ext_truncate()
    freeze_super()  |   starts a handle
    sets s_frozen   |
                    |  ext4_ext_truncate()
                    |  holds i_data_sem
  ext4_freeze()     |
  waits for updates |
                    |  ext4_free_blocks()
                    |  calls dquot_free_block()
                    |
                    |  dquot_free_blocks()
                    |  calls ext4_dirty_inode()
                    |
                    |  ext4_dirty_inode()
                    |  trys to start an active
                    |  handle
                    |
                    |  block due to s_frozen
================================================

Signed-off-by: default avatarYongqiang Yang <xiaoqiangnk@gmail.com>
Signed-off-by: default avatar"Theodore Ts'o" <tytso@mit.edu>
Reported-by: default avatarAmir Goldstein <amir73il@users.sf.net>
Reviewed-by: default avatarJan Kara <jack@suse.cz>
Reviewed-by: default avatarAndreas Dilger <adilger@dilger.ca>
parent 0893ed45

fs/ext4/super.c

+33 −11
Original line number Diff line number Diff line
@@ -242,28 +242,45 @@ static void ext4_put_nojournal(handle_t *handle) 
 * journal_end calls result in the superblock being marked dirty, so

 * that sync() will call the filesystem's write_super callback if

 * appropriate.

 *

 * To avoid j_barrier hold in userspace when a user calls freeze(),

 * ext4 prevents a new handle from being started by s_frozen, which

 * is in an upper layer.

 */

handle_t *ext4_journal_start_sb(struct super_block *sb, int nblocks)

{

	journal_t *journal;

	handle_t  *handle;



	if (sb->s_flags & MS_RDONLY)

		return ERR_PTR(-EROFS);



	journal = EXT4_SB(sb)->s_journal;

	handle = ext4_journal_current_handle();



	/*

	 * If a handle has been started, it should be allowed to

	 * finish, otherwise deadlock could happen between freeze

	 * and others(e.g. truncate) due to the restart of the

	 * journal handle if the filesystem is forzen and active

	 * handles are not stopped.

	 */

	if (!handle)

		vfs_check_frozen(sb, SB_FREEZE_TRANS);

	/* Special case here: if the journal has aborted behind our



	if (!journal)

		return ext4_get_nojournal();

	/*

	 * Special case here: if the journal has aborted behind our

	 * backs (eg. EIO in the commit thread), then we still need to

	 * take the FS itself readonly cleanly. */

	journal = EXT4_SB(sb)->s_journal;

	if (journal) {

	 * take the FS itself readonly cleanly.

	 */

	if (is_journal_aborted(journal)) {

		ext4_abort(sb, "Detected aborted journal");

		return ERR_PTR(-EROFS);

	}

	return jbd2_journal_start(journal, nblocks);

}

	return ext4_get_nojournal();

}



/*

 * The only special thing we need to do here is to make sure that all
@@ -4146,6 +4163,11 @@ static int ext4_sync_fs(struct super_block *sb, int wait) 
/*

 * LVM calls this function before a (read-only) snapshot is created.  This

 * gives us a chance to flush the journal completely and mark the fs clean.

 *

 * Note that only this function cannot bring a filesystem to be in a clean

 * state independently, because ext4 prevents a new handle from being started

 * by @sb->s_frozen, which stays in an upper layer.  It thus needs help from

 * the upper layer.

 */

static int ext4_freeze(struct super_block *sb)

{