On Fri 26-02-21 18:31:03, zhangyi (F) wrote:
> There is an AA deadlock problem when using block_dump on ext4 file
> system with data=journal mode.
>
> watchdog: BUG: soft lockup - CPU#19 stuck for 22s! [jbd2/pmem0-8:1002]
> CPU: 19 PID: 1002 Comm: jbd2/pmem0-8
> RIP: 0010:queued_spin_lock_slowpath+0x60/0x3b0
> ...
> Call Trace:
> _raw_spin_lock+0x57/0x70
> jbd2_journal_invalidatepage+0x166/0x680
> __ext4_journalled_invalidatepage+0x8c/0x120
> ext4_journalled_invalidatepage+0x12/0x40
> truncate_cleanup_page+0x10e/0x1c0
> truncate_inode_pages_range+0x2c8/0xec0
> truncate_inode_pages_final+0x41/0x90
> ext4_evict_inode+0x254/0xac0
> evict+0x11c/0x2f0
> iput+0x20e/0x3a0
> dentry_unlink_inode+0x1bf/0x1d0
> __dentry_kill+0x14c/0x2c0
> dput+0x2bc/0x630
> block_dump___mark_inode_dirty.cold+0x5c/0x111
> __mark_inode_dirty+0x678/0x6b0
> mark_buffer_dirty+0x16e/0x1d0
> __jbd2_journal_temp_unlink_buffer+0x127/0x1f0
> __jbd2_journal_unfile_buffer+0x24/0x80
> __jbd2_journal_refile_buffer+0x12f/0x1b0
> jbd2_journal_commit_transaction+0x244b/0x3030
>
> The problem is a race between jbd2 committing data buffer and user
> unlink the file concurrently. The jbd2 will get jh->b_state_lock and
> redirty the inode's data buffer and inode itself. If block_dump is
> enabled, it will try to find inode's dentry and invoke the last dput()
> after the inode was unlinked. Then the evict procedure will unmap
> buffer and get jh->b_state_lock again in journal_unmap_buffer(), and
> finally lead to deadlock. It works fine if block_dump is not enabled
> because the last evict procedure is not invoked in jbd2 progress and
> the jh->b_state_lock will also prevent inode use after free.
>
> jbd2 xxx
> vfs_unlink
> ext4_unlink
> jbd2_journal_commit_transaction
> **get jh->b_state_lock**
> jbd2_journal_refile_buffer
> mark_buffer_dirty
> __mark_inode_dirty
> block_dump___mark_inode_dirty
> d_find_alias
> d_delete
> unhash
> dput //put the last refcount
> evict
> journal_unmap_buffer
> **get jh->b_state_lock again**
>
> In most cases of where invoking mark_inode_dirty() will get inode's
> refcount and the last iput may not happen, but it's not safe. After
> checking the block_dump code, it only want to dump the file name of the
> dirty inode, so there is no need to get and put denrty, and dump an
> unhashed dentry is also fine. This patch remove the dget() && dput(),
> print the dentry name directly.
>
> Signed-off-by: zhangyi (F) <yi.zhang@xxxxxxxxxx>
> Signed-off-by: yebin (H) <yebin10@xxxxxxxxxx>
Hrm, ok. Honestly, I wanted to just delete that code for a long time. IMO
tracepoints (and we have one in __mark_inode_dirty) are much more useful
for tracing anyway. This code exists only because it was there much before
tracepoints existed... Do you have a strong reason why are you using
block_dump instead of tracepoint trace_writeback_mark_inode_dirty() for
your monitoring?
Honza
> ---
> fs/fs-writeback.c | 20 ++++++++++++++------
> 1 file changed, 14 insertions(+), 6 deletions(-)
>
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index c41cb887eb7d..e9b0952fe236 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -2199,21 +2199,29 @@ int dirtytime_interval_handler(struct ctl_table *table, int write,
> static noinline void block_dump___mark_inode_dirty(struct inode *inode)
> {
> if (inode->i_ino || strcmp(inode->i_sb->s_id, "bdev")) {
> - struct dentry *dentry;
> + struct dentry *dentry = NULL;
> const char *name = "?";
>
> - dentry = d_find_alias(inode);
> - if (dentry) {
> - spin_lock(&dentry->d_lock);
> - name = (const char *) dentry->d_name.name;
> + if (!hlist_empty(&inode->i_dentry)) {
> + spin_lock(&inode->i_lock);
> + if (!hlist_empty(&inode->i_dentry)) {
> + dentry = hlist_entry(inode->i_dentry.first,
> + struct dentry, d_u.d_alias);
> + spin_lock(&dentry->d_lock);
> + name = (const char *) dentry->d_name.name;
> + } else {
> + spin_unlock(&inode->i_lock);
> + }
> }
> +
> printk(KERN_DEBUG
> "%s(%d): dirtied inode %lu (%s) on %s\n",
> current->comm, task_pid_nr(current), inode->i_ino,
> name, inode->i_sb->s_id);
> +
> if (dentry) {
> spin_unlock(&dentry->d_lock);
> - dput(dentry);
> + spin_unlock(&inode->i_lock);
> }
> }
> }
> --
> 2.25.4
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
