netdev - Re: Maybe a race condition in net/rds/rdma.c?

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Date:   Tue, 10 Mar 2020 10:53:38 -0700
From:   santosh.shilimkar@...cle.com
To:     zerons <sironhide0null@...il.com>
Cc:     netdev <netdev@...r.kernel.org>,
        OFED mailing list <linux-rdma@...r.kernel.org>,
        haakon.bugge@...cle.com
Subject: Re: Maybe a race condition in net/rds/rdma.c?

On 3/6/20 4:11 AM, zerons wrote:
> 
> 
> On 2/28/20 02:10, santosh.shilimkar@...cle.com wrote:
>>
>>>> On 18 Feb 2020, at 14:13, zerons <sironhide0null@...il.com> wrote:
>>>>
>>>> Hi, all
>>>>
>>>> In net/rds/rdma.c
>>>> (https://urldefense.com/v3/__https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/net/rds/rdma.c?h=v5.5.3*n419__;Iw!!GqivPVa7Brio!OwwQCLtjDsKmhaIz0sfaOVSuC4ai5t5_FgB7yqNExGOCBtACtIGLF61NNJyqSDtIAcGoPg$ ),
>>>> there may be a race condition between rds_rdma_unuse() and rds_free_mr().
>>>>
>> Hmmm.. I didn't see email before in my inbox. Please post questions/patches on netdev in future which is the correct mailing list.
>>
>>>> It seems that this one need some specific devices to run test,
>>>> unfortunately, I don't have any of these.
>>>> I've already sent two emails to the maintainer for help, no response yet,
>>>> (the email address may not be in use).
>>>>
>>>> 0) in rds_recv_incoming_exthdrs(), it calls rds_rdma_unuse() when receive an
>>>> extension header with force=0, if the victim mr does not have RDS_RDMA_USE_ONCE
>>>> flag set, then the mr would stay in the rbtree. Without any lock, it tries to
>>>> call mr->r_trans->sync_mr().
>>>>
MR won't stay in the rbtree with force flag. If the MR is used or
use_once is set in both cases its removed from the tree.
See "if (mr->r_use_once || force)"

>>>> 1) in rds_free_mr(), the same mr is found, and then freed. The mr->r_refcount
>>>> doesn't change while rds_mr_tree_walk().
>>>>
>>>> 0) back in rds_rdma_unuse(), the victim mr get used again, call
>>>> mr->r_trans->sync_mr().
>>>>
>>>> Could this race condition actually happen?
So from what I see, this race doesn't exist but let me know
if am missing something.

regards,
Santosh