On Tue, Jun 23, 2020 at 07:59:57PM +0200, Marco Elver wrote:
> On Tue, Jun 23, 2020 at 06:37PM +0200, Peter Zijlstra wrote:
> > On Tue, Jun 23, 2020 at 06:13:21PM +0200, Ahmed S. Darwish wrote:
> > > Well, freshly merged code is using it. For example, KCSAN:
> > >
> > > => f1bc96210c6a ("kcsan: Make KCSAN compatible with lockdep")
> > > => kernel/kcsan/report.c:
> > >
> > > void kcsan_report(...)
> > > {
> > > ...
> > > /*
> > > * With TRACE_IRQFLAGS, lockdep's IRQ trace state becomes corrupted if
> > > * we do not turn off lockdep here; this could happen due to recursion
> > > * into lockdep via KCSAN if we detect a race in utilities used by
> > > * lockdep.
> > > */
> > > lockdep_off();
> > > ...
> > > }
> >
> > Marco, do you remember what exactly happened there? Because I'm about to
> > wreck that. That is, I'm going to make TRACE_IRQFLAGS ignore
> > lockdep_off().
>
> Yeah, I was trying to squash any kind of recursion:
>
> lockdep -> other libs ->
> -> KCSAN
> -> print report
> -> dump stack, printk and friends
> -> lockdep -> other libs
> -> KCSAN ...
>
> Some history:
>
> * Initial patch to fix:
> https://lore.kernel.org/lkml/20200115162512.70807-1-elver@xxxxxxxxxx/
That patch is weird; just :=n on lockdep.c should've cured that, the
rest is massive overkill.
> * KCSAN+lockdep+ftrace:
> https://lore.kernel.org/lkml/20200214211035.209972-1-elver@xxxxxxxxxx/
That doesn't really have anything useful..
> lockdep now has KCSAN_SANITIZE := n, but we still need to ensure that
> there are no paths out of lockdep, or the IRQ flags tracing code, that
> might lead through other libs, through KCSAN, libs used to generate a
> report, and back to lockdep.
>
> I never quite figured out the exact trace that led to corruption, but
> avoiding any kind of potential for recursion was the only thing that
> would avoid the check_flags() warnings.
Fair enough; I'll rip it all up and boot a KCSAN kernel, see what if
anything happens.
