Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
In-Reply-To: <20180521153337.GF3803@linux.vnet.ibm.com>
References: <20180518130413.16997-1-roman.penyaev@profitbricks.com>
 <20180518130413.16997-2-roman.penyaev@profitbricks.com> <20180519163735.GX3803@linux.vnet.ibm.com>
 <CAJrWOzAt8BDtypaRwYx_y9jpCVD5W_VL9aQsFsC3bL-XaoJz9g@mail.gmail.com>
 <20180520004318.GY3803@linux.vnet.ibm.com> <CAJrWOzBenkHg_5zs1VAxP_rVbCbOJO_eO0toJBhnzXuy_yWH2Q@mail.gmail.com>
 <CA+55aFymFCVqh4VxtqDPoFYh6zsGEDLSV79_AnsMp3M=t2mhqg@mail.gmail.com> <20180521153337.GF3803@linux.vnet.ibm.com>
From:   Roman Penyaev <roman.penyaev@profitbricks.com>
Date:   Tue, 22 May 2018 11:09:08 +0200
Message-ID: <CAJrWOzDfJ46Vwz5k7VgUwz04EzCCUtt57jxCx1RC0LoOA3t0AA@mail.gmail.com>
Subject: Re: [PATCH v2 01/26] rculist: introduce list_next_or_null_rr_rcu()
To:     "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        linux-block <linux-block@vger.kernel.org>,
        linux-rdma <linux-rdma@vger.kernel.org>,
        Jens Axboe <axboe@kernel.dk>,
        Christoph Hellwig <hch@infradead.org>,
        Sagi Grimberg <sagi@grimberg.me>,
        Bart Van Assche <bart.vanassche@sandisk.com>,
        Or Gerlitz <ogerlitz@mellanox.com>,
        Doug Ledford <dledford@redhat.com>,
        "swapnil.ingle" <swapnil.ingle@profitbricks.com>,
        Danil Kipnis <danil.kipnis@profitbricks.com>,
        Jinpu Wang <jinpu.wang@profitbricks.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, May 21, 2018 at 5:33 PM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
> On Mon, May 21, 2018 at 08:16:59AM -0700, Linus Torvalds wrote:
>> On Mon, May 21, 2018 at 6:51 AM Roman Penyaev <
>> roman.penyaev@profitbricks.com> wrote:
>>
>> > No, I continue from the pointer, which I assigned on the previous IO
>> > in order to send IO fairly and keep load balanced.
>>
>> Right. And that's exactly what has both me and Paul nervous. You're no
>> longer in the RCU domain. You're using a pointer where the lifetime has
>> nothing to do with RCU any more.
>>
>> Can it be done? Sure. But you need *other* locking for it (that you haven't
>> explained), and it's fragile as hell.
>
> He looks to actually have it right, but I would want to see a big comment
> on the read side noting the leak of the pointer and documenting why it
> is OK.

Hi Paul and Linus,

Should I resend current patch with more clear comments about how careful
caller should be with a leaking pointer?  Also I will update read side
with a fat comment about "rcu_assign_pointer()" which leaks the pointer
out of RCU domain and what is done to prevent nasty consequences.
Does that sound acceptable?

--
Roman