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Date:   Tue, 7 Apr 2020 19:52:13 -0700
To:     zerons <>,
        Ka-Cheong Poon <>,
Subject: Re: [PATCH net 2/2] net/rds: Fix MR reference counting problem

On 4/7/20 7:25 PM, zerons wrote:
> 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!
Thanks for confirmation.

Acked-by: Santosh Shilimkar <>

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