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Date:   Mon, 26 Jun 2023 15:09:57 +0200
From:   Jan Kara <jack@suse.cz>
To:     Baokun Li <libaokun1@huawei.com>
Cc:     Jan Kara <jack@suse.cz>, linux-fsdevel@vger.kernel.org,
        linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org,
        yi.zhang@huawei.com, yangerkun@huawei.com, chengzhihao1@huawei.com,
        yukuai3@huawei.com
Subject: Re: [PATCH] quota: fix race condition between dqput() and
 dquot_mark_dquot_dirty()
Message-ID: <20230626130957.kvfli23djxc2opkq@quack3>
References: <20230616085608.42435-1-libaokun1@huawei.com>
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 <c8daf4a0-769f-f769-50f6-8b7063542499@huawei.com>
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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@suse.com>
SUSE Labs, CR