Re: [PATCH v3] ext4: fix circular lock dependency in ext4_ext_migrate

[Zhou, Yun]

> Move iput(tmp_inode) after ext4_writepages_up_write() to avoid a
> circular lock dependency between s_writepages_rwsem and sb_internal
> (freeze protection).
>
> The deadlock scenario:
>
>    CPU0 (EXT4_IOC_MIGRATE)        CPU1 (orphan cleanup during mount)
>    ----                           ----
>    ext4_ext_migrate()
>      ext4_writepages_down_write()
>        s_writepages_rwsem (write)
>                                   ext4_evict_inode()
>                                     sb_start_intwrite()   [sb_internal]
>                                     ...
>                                       ext4_writepages()
>                                         s_writepages_rwsem (read) [BLOCKED]
>      iput(tmp_inode)
>        ext4_evict_inode()
>          sb_start_intwrite()         [BLOCKED]
>
> The tmp_inode is a temporary inode with nlink=0 created solely for
> building the extent tree.  Its eviction does not require
> s_writepages_rwsem protection, so deferring iput() until after
> releasing the rwsem is safe.
>
> Reported-by: syzbot+212e8f62790f8e0bc63b@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=212e8f62790f8e0bc63b
> Fixes: cb85f4d23f79 ("ext4: fix race between writepages and enabling EXT4_EXTENTS_FL")
> Signed-off-by: Yun Zhou <yun.zhou@xxxxxxxxxxxxx>
> Reviewed-by: Jan Kara <jack@xxxxxxx>
> ---
> v3: fixes Reported-by tag and Closes tag.
>
> v2: remove redundant null pointer check for iput(tmp_inode).
>
>   fs/ext4/migrate.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
Hi Honza,

Thank you very much for taking the time to review these patches and 
providing your valuable suggestions. I am eager to solve these 
long-standing deadlock issues on Syzkaller, but I do not have much 
community experience. I'd like to know, regarding this patch, should I 
launch a new RR thread or continue waiting? BR, Yun