linux-kernel - Re: [PATCH v2] blk: improve order of bio handling in generic_make

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Message-ID: <20170310153554.GA24167@redhat.com>
Date:   Fri, 10 Mar 2017 10:35:55 -0500
From:   Mike Snitzer <snitzer@...hat.com>
To:     Jack Wang <jinpu.wang@...fitbricks.com>
Cc:     Mikulas Patocka <mpatocka@...hat.com>,
        Lars Ellenberg <lars.ellenberg@...bit.com>,
        NeilBrown <neilb@...e.com>, Jens Axboe <axboe@...nel.dk>,
        LKML <linux-kernel@...r.kernel.org>,
        Kent Overstreet <kent.overstreet@...il.com>,
        Pavel Machek <pavel@....cz>, linux-raid@...r.kernel.org,
        device-mapper development <dm-devel@...hat.com>,
        linux-block@...r.kernel.org
Subject: Re: [PATCH v2] blk: improve order of bio handling in
 generic_make_request()

On Fri, Mar 10 2017 at 10:07am -0500,
Jack Wang <jinpu.wang@...fitbricks.com> wrote:

> 
> 
> On 10.03.2017 15:55, Mikulas Patocka wrote:
> > 
> > 
> > On Fri, 10 Mar 2017, Mike Snitzer wrote:
> > 
> >> On Fri, Mar 10 2017 at  7:34am -0500,
> >> Lars Ellenberg <lars.ellenberg@...bit.com> wrote:
> >>
> >>>> --- a/block/blk-core.c
> >>>> +++ b/block/blk-core.c
> >>>> @@ -1975,7 +1975,14 @@ generic_make_request_checks(struct bio *bio)
> >>>>   */
> >>>>  blk_qc_t generic_make_request(struct bio *bio)
> >>>>  {
> >>>> -       struct bio_list bio_list_on_stack;
> >>>> +       /*
> >>>> +        * bio_list_on_stack[0] contains bios submitted by the current
> >>>> +        * make_request_fn.
> >>>> +        * bio_list_on_stack[1] contains bios that were submitted before
> >>>> +        * the current make_request_fn, but that haven't been processed
> >>>> +        * yet.
> >>>> +        */
> >>>> +       struct bio_list bio_list_on_stack[2];
> >>>>         blk_qc_t ret = BLK_QC_T_NONE;
> >>>
> >>> May I suggest that, if you intend to assign something that is not a
> >>> plain &(struct bio_list), but a &(struct bio_list[2]),
> >>> you change the task member so it is renamed (current->bio_list vs
> >>> current->bio_lists, plural, is what I did last year).
> >>> Or you will break external modules, silently, and horribly (or,
> >>> rather, they won't notice, but break the kernel).
> >>> Examples of such modules would be DRBD, ZFS, quite possibly others.
> >>
> >> drbd is upstream -- so what is the problem?  (if you are having to
> >> distribute drbd independent of the upstream drbd then why is drbd
> >> upstream?)
> >>
> >> As for ZFS, worrying about ZFS kABI breakage is the last thing we should
> >> be doing.
> > 
> > It's better to make external modules not compile than to silently 
> > introduce bugs in them. So yes, I would rename that.
> > 
> > Mikulas
> 
> Agree, better rename current->bio_list to current->bio_lists

Fine, normally wouldn't do so but I'm not so opposed that we need to get
hung up on this detail.  If Neil and Jens agree then so be it.