From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
To: Jan Kara <jack@suse.cz>
Cc: jack@suse.com, linux-ext4@vger.kernel.org,
linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com,
tytso@mit.edu, mhocko@suse.cz, linux-mm@kvack.org,
syzbot <syzbot+bfdded10ab7dcd7507ae@syzkaller.appspotmail.com>
Subject: Re: possible deadlock in start_this_handle (2)
Date: Mon, 15 Feb 2021 23:06:15 +0900 [thread overview]
Message-ID: <aaee5d61-f988-84c3-4d16-f8b7987f3a83@i-love.sakura.ne.jp> (raw)
In-Reply-To: <20210215124519.GA22417@quack2.suse.cz>
On 2021/02/15 21:45, Jan Kara wrote:
> On Sat 13-02-21 23:26:37, Tetsuo Handa wrote:
>> Excuse me, but it seems to me that nothing prevents
>> ext4_xattr_set_handle() from reaching ext4_xattr_inode_lookup_create()
>> without memalloc_nofs_save() when hitting ext4_get_nojournal() path.
>> Will you explain when ext4_get_nojournal() path is executed?
>
> That's a good question but sadly I don't think that's it.
> ext4_get_nojournal() is called when the filesystem is created without a
> journal. In that case we also don't acquire jbd2_handle lockdep map. In the
> syzbot report we can see:
Since syzbot can test filesystem images, syzbot might have tested a filesystem
image created both with and without journal within this boot.
>
> kswapd0/2246 is trying to acquire lock:
> ffff888041a988e0 (jbd2_handle){++++}-{0:0}, at: start_this_handle+0xf81/0x1380 fs/jbd2/transaction.c:444
>
> but task is already holding lock:
> ffffffff8be892c0 (fs_reclaim){+.+.}-{0:0}, at: __fs_reclaim_acquire+0x0/0x30 mm/page_alloc.c:5195
>
> So this filesystem has very clearly been created with a journal. Also the
> journal lockdep tracking machinery uses:
While locks held by kswapd0/2246 are fs_reclaim, shrinker_rwsem, &type->s_umount_key#38
and jbd2_handle, isn't the dependency lockdep considers problematic is
Chain exists of:
jbd2_handle --> &ei->xattr_sem --> fs_reclaim
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(fs_reclaim);
lock(&ei->xattr_sem);
lock(fs_reclaim);
lock(jbd2_handle);
where CPU0 is kswapd/2246 and CPU1 is the case of ext4_get_nojournal() path?
If someone has taken jbd2_handle and &ei->xattr_sem in this order, isn't this
dependency true?
>
> rwsem_acquire_read(&journal->j_trans_commit_map, 0, 0, _THIS_IP_);
>
> so a lockdep key is per-filesystem. Thus it is not possible that lockdep
> would combine lock dependencies from two different filesystems.
>
> But I guess we could narrow the search for this problem by adding WARN_ONs
> to ext4_xattr_set_handle() and ext4_xattr_inode_lookup_create() like:
>
> WARN_ON(ext4_handle_valid(handle) && !(current->flags & PF_MEMALLOC_NOFS));
>
> It would narrow down a place in which PF_MEMALLOC_NOFS flag isn't set
> properly... At least that seems like the most plausible way forward to me.
You can use CONFIG_DEBUG_AID_FOR_SYZBOT for adding such WARN_ONs on linux-next.
next prev parent reply other threads:[~2021-02-15 14:07 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <000000000000563a0205bafb7970@google.com>
2021-02-11 10:49 ` possible deadlock in start_this_handle (2) Jan Kara
2021-02-11 10:55 ` Michal Hocko
2021-02-11 11:22 ` Dmitry Vyukov
2021-02-11 11:28 ` Dmitry Vyukov
2021-02-11 12:10 ` Jan Kara
2021-02-11 12:34 ` Michal Hocko
2021-02-11 12:57 ` Matthew Wilcox
2021-02-11 13:07 ` Michal Hocko
2021-02-11 13:25 ` Matthew Wilcox
2021-02-11 14:20 ` Michal Hocko
2021-02-11 14:26 ` Matthew Wilcox
2021-02-11 16:41 ` Michal Hocko
2021-02-12 11:18 ` Tetsuo Handa
2021-02-12 12:22 ` Matthew Wilcox
2021-02-12 12:30 ` Michal Hocko
2021-02-12 12:58 ` Tetsuo Handa
2021-02-12 13:12 ` Michal Hocko
2021-02-12 13:34 ` Tetsuo Handa
2021-02-12 15:43 ` Michal Hocko
2021-02-13 10:58 ` Dmitry Vyukov
2021-02-11 13:18 ` Dmitry Vyukov
2021-02-11 11:46 ` Jan Kara
2021-02-13 14:26 ` Tetsuo Handa
2021-02-15 12:45 ` Jan Kara
2021-02-15 14:06 ` Tetsuo Handa [this message]
2021-02-15 14:29 ` Jan Kara
2021-02-19 10:15 ` Tetsuo Handa
2021-02-19 17:22 ` harshad shirwadkar
2021-03-20 10:02 ` Tetsuo Handa
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