Date:   Tue, 1 Mar 2022 22:58:11 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Linus Torvalds' <torvalds@...ux-foundation.org>,
        James Bottomley <James.Bottomley@...senpartnership.com>
CC:     linux-wireless <linux-wireless@...r.kernel.org>,
        "alsa-devel@...a-project.org" <alsa-devel@...a-project.org>,
        KVM list <kvm@...r.kernel.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        "linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
        "nouveau@...ts.freedesktop.org" <nouveau@...ts.freedesktop.org>,
        "Rasmus Villemoes" <linux@...musvillemoes.dk>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Cristiano Giuffrida <c.giuffrida@...nl>,
        "Bos, H.J." <h.j.bos@...nl>,
        "linux1394-devel@...ts.sourceforge.net" 
        <linux1394-devel@...ts.sourceforge.net>,
        "drbd-dev@...ts.linbit.com" <drbd-dev@...ts.linbit.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        CIFS <linux-cifs@...r.kernel.org>,
        "linux-aspeed@...ts.ozlabs.org" <linux-aspeed@...ts.ozlabs.org>,
        linux-scsi <linux-scsi@...r.kernel.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        "linux-staging@...ts.linux.dev" <linux-staging@...ts.linux.dev>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
        "kgdb-bugreport@...ts.sourceforge.net" 
        <kgdb-bugreport@...ts.sourceforge.net>,
        "bcm-kernel-feedback-list@...adcom.com" 
        <bcm-kernel-feedback-list@...adcom.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        "Arnd Bergman" <arnd@...db.de>,
        Linux PM <linux-pm@...r.kernel.org>,
        intel-gfx <intel-gfx@...ts.freedesktop.org>,
        Brian Johannesmeyer <bjohannesmeyer@...il.com>,
        Nathan Chancellor <nathan@...nel.org>,
        dma <dmaengine@...r.kernel.org>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Jakob Koschel <jakobkoschel@...il.com>,
        "v9fs-developer@...ts.sourceforge.net" 
        <v9fs-developer@...ts.sourceforge.net>,
        linux-tegra <linux-tegra@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Andy Shevchenko" <andriy.shevchenko@...ux.intel.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>,
        linux-block <linux-block@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "samba-technical@...ts.samba.org" <samba-technical@...ts.samba.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux F2FS Dev Mailing List 
        <linux-f2fs-devel@...ts.sourceforge.net>,
        "tipc-discussion@...ts.sourceforge.net" 
        <tipc-discussion@...ts.sourceforge.net>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "linux-mediatek@...ts.infradead.org" 
        <linux-mediatek@...ts.infradead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Christian König <christian.koenig@....com>,
        "Mike Rapoport" <rppt@...nel.org>
Subject: RE: [PATCH 2/6] treewide: remove using list iterator after loop body
 as a ptr

From: Linus Torvalds
> Sent: 01 March 2022 19:07
> On Mon, Feb 28, 2022 at 2:29 PM James Bottomley
> <James.Bottomley@...senpartnership.com> wrote:
> >
> > However, if the desire is really to poison the loop variable then we
> > can do
> >
> > #define list_for_each_entry(pos, head, member)                          \
> >         for (pos = list_first_entry(head, typeof(*pos), member);        \
> >              !list_entry_is_head(pos, head, member) && ((pos = NULL) == NULL;                   \
> >              pos = list_next_entry(pos, member))
> >
> > Which would at least set pos to NULL when the loop completes.
> 
> That would actually have been excellent if we had done that
> originally. It would not only avoid the stale and incorrectly typed
> head entry left-over turd, it would also have made it very easy to
> test for "did I find an entry in the loop".
> 
> But I don't much like it in the situation we are now.
> 
> Why? Mainly because it basically changes the semantics of the loop
> _without_ any warnings about it.  And we don't actually get the
> advantage of the nicer semantics, because we can't actually make code
> do
> 
>         list_for_each_entry(entry, ....) {
>                 ..
>         }
>         if (!entry)
>                 return -ESRCH;
>         .. use the entry we found ..
> 
> because that would be a disaster for back-porting, plus it would be a
> flag-day issue (ie we'd have to change the semantics of the loop at
> the same time we change every single user).
> 
> So instead of that simple "if (!entry)", we'd effectively have to
> continue to use something that still works with the old world order
> (ie that "if (list_entry_is_head())" model).
> 
> So we couldn't really take _advantage_ of the nicer semantics, and
> we'd not even get a warning if somebody does it wrong - the code would
> just silently do the wrong thing.
> 
> IOW: I don't think you are wrong about that patch: it would solve the
> problem that Jakob wants to solve, and it would have absolutely been
> much better if we had done this from the beginning. But I think that
> in our current situation, it's actually a really fragile solution to
> the "don't do that then" problem we have.

Can it be resolved by making:
#define list_entry_is_head(pos, head, member) ((pos) == NULL)
and double-checking that it isn't used anywhere else (except in
the list macros themselves).

The odd ones I just found are fs/locks.c mm/page_reporting.c
security/apparmor/apparmorfs.c (3 times)

net/xfrm/xfrm_ipcomp.c#L244 is buggy.
(There is a WARN_ON() then it just carries on regardless!)

There are only about 25 uses of list_entry_is_head().

There are a lot more places where these lists seem to be scanned by hand.
I bet a few of those aren't actually right either.

(Oh at 3am this morning I thought it was a different list type
that could have much the same problem!)

Another plausible solution is a variant of list_foreach_entry()
that does set the 'entry' to NULL at the end.
Then code can be moved over in stages.
I'd reorder the arguments as well as changing the name!

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

Powered by blists - more mailing lists