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

[Baokun Li]

Hello!

Sorry for the late reply, just had a Dragon Boat holiday.

On 2023/6/22 22:56, Jan Kara wrote:
> Hello!
>
> On Mon 19-06-23 14:44:03, Baokun Li wrote:
> > On 2023/6/16 23:28, Jan Kara wrote:
> > > Now calling synchronize_srcu() directly from dquot_transfer() is too
> > > expensive (and mostly unnecessary) so what I would rather suggest is to
> > > create another dquot list (use dq_free list_head inside struct dquot for
> > > it) and add dquot whose last reference should be dropped there. We'd then
> > > queue work item which would call synchronize_srcu() and after that perform
> > > the final cleanup of all the dquots on the list.
> > >
> > > Now this also needs some modifications to dqget() and to quotaoff code to
> > > handle various races with the new dqput() code so if you feel it is too
> > > complex for your taste, I can implement this myself.
> > >
> > > 								Honza
> > I see what you mean, what we are doing here is very similar to
> > drop_dquot_ref(),
> > and if we have to modify it this way, I am happy to implement it.
> >
> > But as you said, calling synchronize_srcu() is too expensive and it blocks
> > almost all
> > mark dirty processes, so we only call it now in performance insensitive
> > scenarios
> > like dquot_disable(). And how do we control how often synchronize_srcu() is
> > called?
> > Are there more than a certain number of dquots in releasing_dquots or are
> > they
> > executed at regular intervals? And it would introduce various new
> > competitions.
> > Is it worthwhile to do this for a corner scenario like this one?
> So the way this is handled (e.g. in fsnotify subsystem) is that we just
> queue work item when we drop the last reference to the protected structure.
> The scheduling latency before the work item gets executed is enough to
> batch synchronize_srcu() calls and once synchronize_srcu() finishes, we add
> all items from the "staging list" to the free_dquots list.

Cool, thanks a lot for clearing up the confusion!

I will implement it in the next version.

> > 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.
>
> 								Honza
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?

Thanks!
--
With Best Regards,
Baokun Li
.