Re: [PATCH] f2fs: quota: fix potential deadlock

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Jan 04, 2022 at 11:05:36PM +0800, Chao Yu wrote:
> On 2022/1/4 21:18, Greg KH wrote:
> > On Tue, Jan 04, 2022 at 09:05:13PM +0800, Chao Yu wrote:
> > > commit a5c0042200b28fff3bde6fa128ddeaef97990f8d upstream.
> > > 
> > > As Yi Zhuang reported in bugzilla:
> > > 
> > > https://bugzilla.kernel.org/show_bug.cgi?id=214299
> > > 
> > > There is potential deadlock during quota data flush as below:
> > > 
> > > Thread A:			Thread B:
> > > f2fs_dquot_acquire
> > > down_read(&sbi->quota_sem)
> > > 				f2fs_write_checkpoint
> > > 				block_operations
> > > 				f2fs_look_all
> > > 				down_write(&sbi->cp_rwsem)
> > > f2fs_quota_write
> > > f2fs_write_begin
> > > __do_map_lock
> > > f2fs_lock_op
> > > down_read(&sbi->cp_rwsem)
> > > 				__need_flush_qutoa
> > > 				down_write(&sbi->quota_sem)
> > > 
> > > This patch changes block_operations() to use trylock, if it fails,
> > > it means there is potential quota data updater, in this condition,
> > > let's flush quota data first and then trylock again to check dirty
> > > status of quota data.
> > > 
> > > The side effect is: in heavy race condition (e.g. multi quota data
> > > upaters vs quota data flusher), it may decrease the probability of
> > > synchronizing quota data successfully in checkpoint() due to limited
> > > retry time of quota flush.
> > > 
> > > Fixes: db6ec53b7e03 ("f2fs: add a rw_sem to cover quota flag changes")
> > > Cc: stable@vger.kernel.org # v5.3+
> > > Reported-by: Yi Zhuang <zhuangyi1@huawei.com>
> > > Signed-off-by: Chao Yu <chao@kernel.org>
> > > Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
> > > ---
> > >   fs/f2fs/checkpoint.c | 3 ++-
> > >   1 file changed, 2 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
> > > index 83e9bc0f91ff..7b0282724231 100644
> > > --- a/fs/f2fs/checkpoint.c
> > > +++ b/fs/f2fs/checkpoint.c
> > > @@ -1162,7 +1162,8 @@ static bool __need_flush_quota(struct f2fs_sb_info *sbi)
> > >   	if (!is_journalled_quota(sbi))
> > >   		return false;
> > > -	down_write(&sbi->quota_sem);
> > > +	if (!down_write_trylock(&sbi->quota_sem))
> > > +		return true;
> > >   	if (is_sbi_flag_set(sbi, SBI_QUOTA_SKIP_FLUSH)) {
> > >   		ret = false;
> > >   	} else if (is_sbi_flag_set(sbi, SBI_QUOTA_NEED_REPAIR)) {
> > > -- 
> > > 2.32.0
> > > 
> > 
> > What stable tree(s) is this for?
> 
> Oh, please help to try applying to 5.4, 5.10, and 5.15 stable trees, thanks!

This is already in the 5.15.6 kernel release, do you need it applied
there again?  :)

> Let me know if I should send patches for different trees separately.

If the same commit here works for all of the above, it's fine.  But for
some reason I don't think it will work in 5.15.y...

thanks,

greg k-h