Messages in this thread | ![/](/images/icornerl.gif) | | Date | Mon, 26 Jun 2023 21:55:49 +0800 | Subject | Re: [PATCH] quota: fix race condition between dqput() and dquot_mark_dquot_dirty() | From | Baokun Li <> |
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Hello!
On 2023/6/26 21:09, Jan Kara wrote: > 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 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.
-- With Best Regards, Baokun Li .
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