linux-kernel - Re: [PATCH v3] riscv: patch: Fixup lockdep warning in stop

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Thu, 2 Feb 2023 08:01:44 +0000
From:   Conor Dooley <conor.dooley@...rochip.com>
To:     Changbin Du <changbin.du@...wei.com>
CC:     Guo Ren <guoren@...nel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Hui Wang <hw.huiwang@...wei.com>,
        <linux-riscv@...ts.infradead.org>, <linux-kernel@...r.kernel.org>,
        Changbin Du <changbin.du@...il.com>,
        Zong Li <zong.li@...ive.com>
Subject: Re: [PATCH v3] riscv: patch: Fixup lockdep warning in stop_machine

On Fri, Feb 03, 2023 at 07:00:43AM +0800, Changbin Du wrote:

btw, something is wrong with your mail client or host machine.
Everything that you are sending is timestamped in the future,
as it is currently 15:57 on the 2nd in UTC+8.

> 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@rivosinc.com/

Oh cool, thanks for that.
Why not resend that approach, with your suggested fixup for
ftrace_init_nop() then?
It looks more complex, but is less worrisome & has an R-b from Steven
already.

Thanks,
Conor.


Download attachment "signature.asc" of type "application/pgp-signature" (229 bytes)