Re: [PATCH] quota: fix race condition between dqput() and dquot_mark_dquot_dirty()

[Jan Kara]

Hello!

On Sun 25-06-23 15:56:10, Baokun Li wrote:
> > > I think we can simply focus on the race between the DQ_ACTIVE_B flag and
> > > the DQ_MOD_B flag, which is the core problem, because the same quota
> > > should not have both flags. These two flags are protected by dq_list_lock
> > > and dquot->dq_lock respectively, so it makes sense to add a
> > > wait_on_dquot() to ensure the accuracy of DQ_ACTIVE_B.
> > But the fundamental problem is not only the race with DQ_MOD_B setting. The
> > dquot structure can be completely freed by the time
> > dquot_claim_space_nodirty() calls dquot_mark_dquot_dirty() on it. That's
> > why I think making __dquot_transfer() obey dquot_srcu rules is the right
> > solution.
> Yes, now I also think that making __dquot_transfer() obey dquot_srcu
> rules is a better solution. But with inode->i_lock protection, why would
> the dquot structure be completely freed?

Well, when dquot_claim_space_nodirty() calls mark_all_dquot_dirty() it does
not hold any locks (only dquot_srcu). So nothing prevents dquot_transfer()
to go, swap dquot structure pointers and drop dquot references and after
that mark_all_dquot_dirty() can use a stale pointer to call
mark_dquot_dirty() on already freed memory.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR