Date:   Tue, 27 Jun 2023 11:28:29 +0200
From:   Jan Kara <jack@...e.cz>
To:     Baokun Li <libaokun1@...wei.com>
Cc:     Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com, yangerkun@...wei.com, chengzhihao1@...wei.com,
        yukuai3@...wei.com
Subject: Re: [PATCH] quota: fix race condition between dqput() and
 dquot_mark_dquot_dirty()

On Tue 27-06-23 17:08:27, Baokun Li wrote:
> Hello!
> 
> On 2023/6/27 16:34, Jan Kara wrote:
> > Hello!
> > 
> > On Mon 26-06-23 21:55:49, Baokun Li wrote:
> > > On 2023/6/26 21:09, Jan Kara wrote:
> > > > 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.
> > > > 
> > > No, this doesn't look like it's going to happen.  The
> > > mark_all_dquot_dirty() uses a pointer array pointer, the dquot in the
> > > array is dynamically changing, so after swap dquot structure pointers,
> > > mark_all_dquot_dirty() uses the new pointer, and the stale pointer is
> > > always destroyed after swap, so there is no case of using the stale
> > > pointer here.
> > There is a case - CPU0 can prefetch the values from dquots[] array into its
> > local cache, then CPU1 can update the dquots[] array (these writes can
> > happily stay in CPU1 store cache invisible to other CPUs) and free the
> > dquots via dqput(). Then CPU0 can pass the prefetched dquot pointers to
> > mark_dquot_dirty(). There are no locks or memory barries preventing CPUs
> > from ordering instructions and memory operations like this in the code...
> > You can read Documentation/memory-barriers.txt about all the perils current
> > CPU architecture brings wrt coordination of memory accesses among CPUs ;)
> > 
> > 								Honza
> 
> Got it!
> 
> Sorry for misunderstanding you (I thought "completely freed" meant
> dquot_destroy(), but you should have meant dquot_release()).

Well, the dquot can even get to dquot_destroy(). There's nothing really
preventing CPU2 going into memory reclaim and free the dquot in
dqcache_shrink_scan() still before CPU0 even calls mark_dquot_dirty() on
it. Sure such timing on real hardware is very unlikely but in a VM where a
virtual CPU can get starved for a significant amount of time this could
happen.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR

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