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Date:   Wed, 8 Apr 2020 10:25:11 +0800
From:   zerons <>
        Ka-Cheong Poon <>,
Subject: Re: [PATCH net 2/2] net/rds: Fix MR reference counting problem

On 4/8/20 03:30, wrote:
> On 4/7/20 9:08 AM, Ka-Cheong Poon wrote:
>> In rds_free_mr(), it calls rds_destroy_mr(mr) directly.  But this
>> defeats the purpose of reference counting and makes MR free handling
>> impossible.  It means that holding a reference does not guarantee that
>> it is safe to access some fields.  For example, In
>> rds_cmsg_rdma_dest(), it increases the ref count, unlocks and then
>> calls mr->r_trans->sync_mr().  But if rds_free_mr() (and
>> rds_destroy_mr()) is called in between (there is no lock preventing
>> this to happen), r_trans_private is set to NULL, causing a panic.
>> Similar issue is in rds_rdma_unuse().
>> Reported-by: zerons <>
>> Signed-off-by: Ka-Cheong Poon <>
>> ---
> Thanks for getting this out on the list.
> Hi zerons,
> Can you please review it and see it addresses your concern ?

Yes, the MR gets freed only when the ref count decreases to zero does
address my concern. I think it make the logic cleaner as well. Fantastic!


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