Re: [PATCH v2 0/8] Support Clang context analysis for ext2

[Timothy Day <timday@thelustrecollective.com>]

On Thu, 03 Sep 2026 14:43:03 +0200, Jan Kara wrote:
> On Thu 03-09-26 14:06:22, Marco Elver wrote:
> > On Thu, 3 Sept 2026 at 13:06, Jan Kara <jack@suse.cz> wrote:
> > >
> > > On Thu 03-09-26 12:34:34, Marco Elver wrote:
> > > > On Thu, 3 Sept 2026 at 09:28, Nathan Chancellor <nathan@kernel.org> wrote:
> > > > >   fs/ext2/super.c:1149:3: error: calling function 'ext2_rsv_window_add' requires holding spinlock 'EXT2_SB(sb).s_rsv_window_lock' exclusively [-Werror,-Wthread-safety-precise]
> > > > >    1149 |                 ext2_rsv_window_add(sb, &sbi->s_rsv_window_head);
> > > > >         |                 ^
> > > > >   fs/ext2/super.c:1149:3: note: found near match '_res->s_rsv_window_lock'
> > > >
> > > > sbi was just allocated, and EXT2_SB(sb) and sbi are pointing to the
> > > > same object (unless I misread the code), but the compiler can't tell
> > > > since aliases can't be tracked through non-local objects. So that
> > > > scoped_guard could just become:
> > > >
> > > >   scoped_guard(spinlock, &EXT2_SB(sb)->s_rsv_window_lock) {
> > > >   ...
> > > >
> > > > But I'll leave that to Jan and Tim.
> > >
> > > Yes, at the beginning of ext4_fill_super() we do:
> > >
> > >         sbi = kzalloc_obj(*sbi);
> > >         ...
> > >         sb->s_fs_info = sbi;
> > >
> > > and EXT2_SB(sb) is just sb->s_fs_info. Are you saying that clang is not
> > > able to infer that sb->s_fs_info and sbi are still pointing to the same
> > > memory later in the function where we do scoped_guard()?
> >
> > Yes - alias tracking is only done through local aliases. sb is
> > non-local, along with additional member indirection; unfortunately,
> > the C language doesn't give us the guarantees that it wasn't modified
> > somewhere in between, say after a function call (this rule is applied
> > also for local aliases if clang sees that they "escape" their local
> > scope via non-const pointer to pointer).
>
> Hrm, ok, understood (but still it's annoying ;)). I've pushed out the patch
> with the suggested fixup.

Sorry for the late reply. I was out last month due to lung trouble
(spontaneous pneumothorax corrected via surgery). Thanks Jan for
pulling the series. I took a look at your branch [2] and everything
seems fine. I agree the scoped guards can be peculiar. If you
wanted to make some minor changes, I have no objection.

Thanks Marco for the compilation fix in the other thread [1]. It seems
like a reasonable fix. If there is anything else needed from me, let
me know.

Now that I'm back, I'm hoping to resume work on some more context
analysis patches. I have a few that were close to done that I
hope to share soon.

Tim Day

[1] https://lore.kernel.org/all/20260903101843.3462767-1-elver@google.com/
[2] https://git.kernel.org/pub/scm/linux/kernel/git/jack/linux-fs.git/log/?h=for_next