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Darwish" , sparclinux@vger.kernel.org, linux@armlinux.org.uk, tglx@linutronix.de, will@kernel.org, mingo@kernel.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" 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@google.com/ 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@google.com/ 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.