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Subject: Re: [PATCH 2/6] treewide: remove using list iterator after loop
 body as a ptr
From:   James Bottomley <James.Bottomley@HansenPartnership.com>
To:     Christian =?ISO-8859-1?Q?K=F6nig?= <christian.koenig@amd.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Jakob Koschel <jakobkoschel@gmail.com>,
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Date:   Mon, 28 Feb 2022 16:13:09 -0500
In-Reply-To: <0b65541a-3da7-dc35-690a-0ada75b0adae@amd.com>
References: <20220228110822.491923-1-jakobkoschel@gmail.com>
         <20220228110822.491923-3-jakobkoschel@gmail.com>
         <2e4e95d6-f6c9-a188-e1cd-b1eae465562a@amd.com>
         <CAHk-=wgQps58DPEOe4y5cTh5oE9EdNTWRLXzgMiETc+mFX7jzw@mail.gmail.com>
         <282f0f8d-f491-26fc-6ae0-604b367a5a1a@amd.com>
         <b2d20961dbb7533f380827a7fcc313ff849875c1.camel@HansenPartnership.com>
         <0b65541a-3da7-dc35-690a-0ada75b0adae@amd.com>
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On Mon, 2022-02-28 at 21:56 +0100, Christian König wrote:
> 
> Am 28.02.22 um 21:42 schrieb James Bottomley:
> > On Mon, 2022-02-28 at 21:07 +0100, Christian König wrote:
> > > Am 28.02.22 um 20:56 schrieb Linus Torvalds:
> > > > On Mon, Feb 28, 2022 at 4:19 AM Christian König
> > > > <christian.koenig@amd.com> wrote:
> > > > [SNIP]
> > > > Anybody have any ideas?
> > > I think we should look at the use cases why code is touching
> > > (pos)
> > > after the loop.
> > > 
> > > Just from skimming over the patches to change this and experience
> > > with the drivers/subsystems I help to maintain I think the
> > > primary pattern looks something like this:
> > > 
> > > list_for_each_entry(entry, head, member) {
> > >       if (some_condition_checking(entry))
> > >           break;
> > > }
> > > do_something_with(entry);
> > 
> > Actually, we usually have a check to see if the loop found
> > anything, but in that case it should something like
> > 
> > if (list_entry_is_head(entry, head, member)) {
> >      return with error;
> > }
> > do_somethin_with(entry);
> > 
> > Suffice?  The list_entry_is_head() macro is designed to cope with
> > the bogus entry on head problem.
> 
> That will work and is also what people already do.
> 
> The key problem is that we let people do the same thing over and
> over again with slightly different implementations.
> 
> Out in the wild I've seen at least using a separate variable, using
> a bool to indicate that something was found and just assuming that
> the list has an entry.
> 
> The last case is bogus and basically what can break badly.

Yes, I understand that.  I'm saying we should replace that bogus checks
of entry->something against some_value loop termination condition with
the list_entry_is_head() macro.  That should be a one line and fairly
mechanical change rather than the explosion of code changes we seem to
have in the patch series.

James