linux-kernel - Re: [PATCH v2 01/26] rculist: introduce list_next_or_null_rr

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Message-ID: <CAJrWOzDfJ46Vwz5k7VgUwz04EzCCUtt57jxCx1RC0LoOA3t0AA@mail.gmail.com>
Date:   Tue, 22 May 2018 11:09:08 +0200
From:   Roman Penyaev <roman.penyaev@...fitbricks.com>
To:     "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-block <linux-block@...r.kernel.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>,
        Christoph Hellwig <hch@...radead.org>,
        Sagi Grimberg <sagi@...mberg.me>,
        Bart Van Assche <bart.vanassche@...disk.com>,
        Or Gerlitz <ogerlitz@...lanox.com>,
        Doug Ledford <dledford@...hat.com>,
        "swapnil.ingle" <swapnil.ingle@...fitbricks.com>,
        Danil Kipnis <danil.kipnis@...fitbricks.com>,
        Jinpu Wang <jinpu.wang@...fitbricks.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 01/26] rculist: introduce list_next_or_null_rr_rcu()

On Mon, May 21, 2018 at 5:33 PM, Paul E. McKenney
<paulmck@...ux.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@...fitbricks.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