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Date: Tue, 18 Aug 2020 17:41:43 +0200
From: peterz@infradead.org
To: Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH 1/2] lockdep: improve current->(hard|soft)irqs_enabled
 synchronisation with actual irq state
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Cc: linux-arch@vger.kernel.org, Alexey Kardashevskiy <aik@ozlabs.ru>,
 Will Deacon <will@kernel.org>, linux-kernel@vger.kernel.org,
 Ingo Molnar <mingo@redhat.com>, linuxppc-dev@lists.ozlabs.org
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

On Tue, Aug 18, 2020 at 05:22:33PM +1000, Nicholas Piggin wrote:
> Excerpts from peterz@infradead.org's message of August 12, 2020 8:35 pm:
> > On Wed, Aug 12, 2020 at 06:18:28PM +1000, Nicholas Piggin wrote:
> >> Excerpts from peterz@infradead.org's message of August 7, 2020 9:11 pm:
> >> > 
> >> > What's wrong with something like this?
> >> > 
> >> > AFAICT there's no reason to actually try and add IRQ tracing here, it's
> >> > just a hand full of instructions at the most.
> >> 
> >> Because we may want to use that in other places as well, so it would
> >> be nice to have tracing.
> >> 
> >> Hmm... also, I thought NMI context was free to call local_irq_save/restore
> >> anyway so the bug would still be there in those cases?
> > 
> > NMI code has in_nmi() true, in which case the IRQ tracing is disabled
> > (except for x86 which has CONFIG_TRACE_IRQFLAGS_NMI).
> > 
> 
> That doesn't help. It doesn't fix the lockdep irq state going out of
> synch with the actual irq state. The code which triggered this with the
> special powerpc irq disable has in_nmi() true as well.

Urgh, you're talking about using lockdep_assert_irqs*() from NMI
context?

If not, I'm afraid I might've lost the plot a little on what exact
failure case we're talking about.