Re: [PATCH RFC 2/3] locking/rwsem: clear reader-owner on unlock to reduce false positives

[Lance Yang]

On 2025/6/24 14:13, Masami Hiramatsu (Google) wrote:
> On Tue, 24 Jun 2025 13:02:31 +0800
> Lance Yang <lance.yang@xxxxxxxxx> wrote:
>
> > On 2025/6/24 11:53, Masami Hiramatsu (Google) wrote:
> > > On Tue, 24 Jun 2025 09:44:55 +0800
> > > Lance Yang <lance.yang@xxxxxxxxx> wrote:
> > >
> > > > On 2025/6/24 08:26, Masami Hiramatsu (Google) wrote:
> > > > > On Thu, 12 Jun 2025 12:19:25 +0800
> > > > > Lance Yang <ioworker0@xxxxxxxxx> wrote:
> > > > >
> > > > > > From: Lance Yang <lance.yang@xxxxxxxxx>
> > > > > >
> > > > > > When CONFIG_DETECT_HUNG_TASK_BLOCKER is enabled, a stale owner pointer in a
> > > > > > reader-owned rwsem can lead to false positives in blocker tracking.
> > > > > >
> > > > > > To mitigate this, let’s try to clear the owner field on unlock, as a NULL
> > > > > > owner is better than a stale one for diagnostics.
> > > > >
> > > > > Can we merge this to [PATCH 1/3]? It seems that you removed #ifdef and
> > > > > remove it. This means in anyway we need the feature enabled by DEBUG_RWSEMS.
> > > >
> > > > Thanks for the feedback! I see your point about the dependency ;)
> > > >
> > > > Personlly, I'd perfer to keep them separate. The reasoning is that
> > > > they addreess two distinct things, and I think splitting them makes
> > > > this series clearer and easier to review ;)
> > > >
> > > > Patch #1 focuses on "ownership tracking": Its only job is to make
> > > > the existing owner-related helpers (rwsem_owner(), is_rwsem_reader_owned())
> > > > globally available when blocker tracking is enabled.
> > > >
> > > > Patch #2, on the other hand, is about "reader-owner cleanup": It
> > > > introduces a functional change to the unlock path, trying to clear
> > > > the owner field for reader-owned rwsems.
> > >
> > > But without clearing the owner, the owner information can be
> > > broken, right? Since CONFIG_DEBUG_RWSEMS is working as it is,
> >
> > You're right, the owner info would be broken without the cleanup logic
> > in patch #2. But ...
> >
> > > I think those cannot be decoupled. For example, comparing the
> > > result of both DETECT_HUNG_TASK_BLOCKER and DEBUG_RWSEMS are
> > > enabled and only DETECT_HUNG_TASK_BLOCKER is enabled, the
> > > result is different.
> >
> > The actual blocker tracking for rwsems is only turned on in patch #3.
> > So, there's no case where the feature is active without the cleanup
> > logic already being in place.
> >
> > > > Does this reasoning make sense to you?
> > >
> > > Sorry, no. I think "reader-owner cleanup" is a part of "ownership
> > > tracking" as DEBUG_RWSEMS does (and that keeps consistency of
> > > the ownership tracking behavior same as DEBUG_RWSEM).
> >
> > I thought this step-by-step approach was a bit cleaner, since there are
> > currently only two users for these owner helpers (DEBUG_RWSEMS and
> > DETECT_HUNG_TASK_BLOCKER).
>
> I think the step-by-step approach fits better if the feature is evolving
> (a working feature is already there.) I don't like the intermediate

Agreed.

> state which does not work correctly, because if we have a unit test(
> like kUnit) it should fail. If you can say "this finds the rwsem

Ah, I missed that ...

> owner as same as what the CONFIG_DEBUG_RWSEM is doing", it is simpler
> to explain what you are doing, and easy to understand.

Thanks for the lesson! Will rework the series as you suggested ;)
Lance