v2->v1:
- rearrange code.
v0->v1:
- check s_frozen in nojournal case only if there is no an active handle.
- get reference to an active handle by jbd2_journal_start().
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 <xiaoqiangnk@xxxxxxxxx>
Reported-by: Amir Goldstein <amir73il@xxxxxxxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
Reviewed-by: Andreas Dilger <adilger@xxxxxxxxx>
Signed-off-by: Yongqiang Yang <xiaoqiangnk@xxxxxxxxx>
---
fs/ext4/super.c | 44 +++++++++++++++++++++++++++++++++-----------
1 files changed, 33 insertions(+), 11 deletions(-)
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index ccfa686..7fc6c2a 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);
}
/*
@@ -4131,6 +4148,11 @@ 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)
{
--
1.7.4
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