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

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Date:   Fri, 10 Mar 2017 09:55:01 -0500 (EST)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Mike Snitzer <snitzer@...hat.com>
cc:     Lars Ellenberg <lars.ellenberg@...bit.com>,
        NeilBrown <neilb@...e.com>, Jens Axboe <axboe@...nel.dk>,
        Jack Wang <jinpu.wang@...fitbricks.com>,
        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, 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

> So Nack from me on this defensive make-work for external modules.
>