Re: [PATCH v3] riscv: patch: Fixup lockdep warning in stop_machine

[Changbin Du]

On Wed, Feb 01, 2023 at 02:01:07PM +0000, Conor Dooley wrote:
> On Thu, Feb 02, 2023 at 05:00:31AM +0800, Changbin Du wrote:
> > On Tue, Jan 31, 2023 at 07:50:20AM +0000, Conor Dooley wrote:
> > > On Tue, Jan 31, 2023 at 03:26:33PM +0800, Guo Ren wrote:
> > [snip]
> > > > > >
> > > > > > -     /*
> > > > > > -      * Before reaching here, it was expected to lock the text_mutex
> > > > > > -      * already, so we don't need to give another lock here and could
> > > > > > -      * ensure that it was safe between each cores.
> > > > > > -      */
> > > > > > -     lockdep_assert_held(&text_mutex);
> > > > >
> > > > > I must admit, patches like this do concern me a little, as a someone
> > > > > unfamiliar with the world of probing and tracing.
> > > > > Seeing an explicit check that the lock was held, leads me to believe
> > > > > that the original author (Zong Li I think) thought that the text_mutex
> > > > > lock was insufficient.
> > > > > Do you think that their fear is unfounded? Explaining why it is safe to
> > > > > remove this assertion in the commit message would go a long way towards
> > > > > easing my anxiety!
> > > > >
> > > > > Also, why delete the comment altogether? The comment provides some
> > > > > information that doesn't appear to become invalid, even with the
> > > > > assertion removed?
> > > > Stop_machine separated the mutex context and made a lockdep warning.
> > > > So text_mutex can't be used here. We need to find another check
> > > > solution. I agree with the patch.
> > > 
> > > Whether or not you agree with the change is not the point (with your SoB
> > > I'd hope you agree with it).
> > > I understand that you two are trying to fix a false positive lockdep
> > > warning, but what I am asking for an explanation as to why the original
> > > author's fear is unfounded.
> > > Surely, having added the assertion, they were not thinking of the same
> > > code path that you guys are hitting the false positive on?
> > > 
> > The assertion is reasonable since the fixmap entry is shared. The text_mutex
> > does should be held before entering that function. But the false positive cases
> > make some functions (ftrace for example) difficult to use due to warning log
> > storm.
> > 
> > Either the lockdep should be fixed for stop_machine, or remove the assertion
> > simply now (we can keep the comments). (or do the assertion conditionly?)
> 
> How would you suggest checking it conditionally?
>
Please refer to a early patch from Palmer Dabbelt.
https://lore.kernel.org/all/20220322022331.32136-1-palmer@xxxxxxxxxxxx/

> Also, if you persist in removing the assertion, there is a comment in
> arch/riscv/kernel/ftrace.c that would need to be updated. (L129-ish)
> 
No problem.

> The comment you removed in this patch seems valid both before and after
> though, so I don't see a compelling reason for its removal.
We all agreed. The key is to get rid of false positive case.

> 
> > And this is not a riscv only problem but common for architectures which use
> > stop_machine to patch text. (arm for example)
> > 
> > > Perhaps Zong themselves can tell us what the original fear was?
> 



-- 
Cheers,
Changbin Du