Re: [PATCH 2/6] treewide: remove using list iterator after loop body as a ptr

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:

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On February 28, 2022 10:42:53 PM GMT+02:00, James Bottomley <
James.Bottomley@HansenPartnership.com> wrote:

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On Mon, 2022-02-28 at 21:07 +0100, Christian König wrote:

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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