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Date:   Mon, 28 Feb 2022 18:30:59 -0600
From:   Segher Boessenkool <segher@kernel.crashing.org>
To:     James Bottomley <James.Bottomley@HansenPartnership.com>
Cc:     Mike Rapoport <rppt@kernel.org>,
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Subject: Re: [PATCH 2/6] treewide: remove using list iterator after loop body as a ptr
Message-ID: <20220301003059.GE614@gate.crashing.org>
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> <7D0C2A5D-500E-4F38-AD0C-A76E132A390E@kernel.org> <73fa82a20910c06784be2352a655acc59e9942ea.camel@HansenPartnership.com>
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On Mon, Feb 28, 2022 at 05:28:58PM -0500, James Bottomley wrote:
> On Mon, 2022-02-28 at 23:59 +0200, Mike Rapoport wrote:
> > 
> > On February 28, 2022 10:42:53 PM GMT+02:00, James Bottomley <
> > James.Bottomley@HansenPartnership.com> wrote:
> > > On Mon, 2022-02-28 at 21:07 +0100, Christian K?nig wrote:
> [...]
> > > > > I do wish we could actually poison the 'pos' value after the
> > > > > loop somehow - but clearly the "might be uninitialized" I was
> > > > > hoping for isn't the way to do it.
> > > > > 
> > > > > 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.
> > 
> > Won't suffice because the end goal of this work is to limit scope of
> > entry only to loop. Hence the need for additional variable.
> 
> Well, yes, but my objection is more to the size of churn than the
> desire to do loop local.  I'm not even sure loop local is possible,
> because it's always annoyed me that for (int i = 0; ...  in C++ defines
> i in the outer scope not the loop scope, which is why I never use it.

In C its scope is the rest of the declaration and the entire loop, not
anything after it.  This was the same in C++98 already, btw (but in
pre-standard versions of C++ things were like you remember, yes, and it
was painful).


Segher