From be4f27d324e8ddd57cc0d4d604fe85ee0425cba9 Mon Sep 17 00:00:00 2001 From: Yongqiang Yang Date: Sun, 10 Apr 2011 22:06:07 -0400 Subject: [PATCH] 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: Yongqiang Yang Signed-off-by: "Theodore Ts'o" Reported-by: Amir Goldstein Reviewed-by: Jan Kara Reviewed-by: Andreas Dilger --- fs/ext4/super.c | 44 +++++++++++++++++++++++++++++++++----------- 1 file changed, 33 insertions(+), 11 deletions(-) diff --git a/fs/ext4/super.c b/fs/ext4/super.c index 551cb8e2110c..7b636ceb878a 100644 --- a/fs/ext4/super.c +++ b/fs/ext4/super.c @@ -242,27 +242,44 @@ 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); - vfs_check_frozen(sb, SB_FREEZE_TRANS); - /* 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) { - if (is_journal_aborted(journal)) { - ext4_abort(sb, "Detected aborted journal"); - return ERR_PTR(-EROFS); - } - return jbd2_journal_start(journal, nblocks); + 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); + + 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. + */ + if (is_journal_aborted(journal)) { + ext4_abort(sb, "Detected aborted journal"); + return ERR_PTR(-EROFS); } - return ext4_get_nojournal(); + return jbd2_journal_start(journal, nblocks); } /* @@ -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) { -- 2.30.2