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

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Message-Id: <20180522163648.GV3803@linux.vnet.ibm.com>
Date:   Tue, 22 May 2018 09:36:48 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Roman Penyaev <roman.penyaev@...fitbricks.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 Tue, May 22, 2018 at 11:09:08AM +0200, Roman Penyaev wrote:
> 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?

That sounds good to me.

							Thanx, Paul