From: peterz@infradead.org
To: Nicholas Piggin <npiggin@gmail.com>
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
Subject: Re: [PATCH 1/2] lockdep: improve current->(hard|soft)irqs_enabled synchronisation with actual irq state
Date: Sun, 26 Jul 2020 14:11:38 +0200 [thread overview]
Message-ID: <20200726121138.GC119549@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <1595735694.b784cvipam.astroid@bobo.none>
On Sun, Jul 26, 2020 at 02:14:34PM +1000, Nicholas Piggin wrote:
> Excerpts from Peter Zijlstra's message of July 26, 2020 6:26 am:
> > Which is 'funny' when it interleaves like:
> >
> > local_irq_disable();
> > ...
> > local_irq_enable()
> > trace_hardirqs_on();
> > <NMI/>
> > raw_local_irq_enable();
> >
> > Because then it will undo the trace_hardirqs_on() we just did. With the
> > result that both tracing and lockdep will see a hardirqs-disable without
> > a matching enable, while the hardware state is enabled.
>
> Seems like an arch problem -- why not disable if it was enabled only?
> I guess the local_irq tracing calls are a mess so maybe they copied
> those.
Because, as I wrote earlier, then we can miss updating software state.
So your proposal has:
raw_local_irq_disable()
<NMI>
if (!arch_irqs_disabled(regs->flags) // false
trace_hardirqs_off();
// tracing/lockdep still think IRQs are enabled
// hardware IRQ state is disabled.
With the current code we have:
local_irq_enable()
trace_hardirqs_on();
<NMI>
trace_hardirqs_off();
...
if (!arch_irqs_disabled(regs->flags)) // false
trace_hardirqs_on();
</NMI>
// and now the NMI disabled software state again
// while we're about to enable the hardware state
raw_local_irq_enable();
> > Which is exactly the state Alexey seems to have ran into.
>
> No his was what I said, the interruptee's trace_hardirqs_on() in
> local_irq_enable getting lost because the NMI's local_irq_disable
> always disables, but the enable doesn't re-enable.
That's _exactly_ the case above. It doesn't re-enable because hardirqs
are actually still disabled. You _cannot_ rely on hardirq state for
NMIs, that'll get you wrong state.
> It's all just weird asymmetrical special case hacks AFAIKS, the
> code should just be symmetric and lockdep handle it's own weirdness.
It's for non-maskable exceptions/interrupts, because there the hardware
and software state changes non-atomically. For maskable interrupts doing
the software state transitions inside the disabled region makes perfect
sense, because that keeps it atomic.
next prev parent reply other threads:[~2020-07-26 12:13 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-23 10:56 [PATCH 1/2] lockdep: improve current->(hard|soft)irqs_enabled synchronisation with actual irq state Nicholas Piggin
2020-07-23 10:56 ` [PATCH 2/2] lockdep: warn on redundant or incorrect irq state changes Nicholas Piggin
2020-07-24 2:57 ` kernel test robot
2020-08-04 10:00 ` Alexey Kardashevskiy
2020-07-23 11:40 ` [PATCH 1/2] lockdep: improve current->(hard|soft)irqs_enabled synchronisation with actual irq state Peter Zijlstra
2020-07-23 13:11 ` Nicholas Piggin
2020-07-23 14:59 ` Peter Zijlstra
2020-07-23 16:20 ` Nicholas Piggin
2020-07-24 4:16 ` Alexey Kardashevskiy
2020-07-24 5:59 ` Nicholas Piggin
2020-07-26 7:40 ` Alexey Kardashevskiy
2020-07-24 6:16 ` Athira Rajeev
2020-07-24 2:19 ` kernel test robot
2020-07-24 3:15 ` kernel test robot
2020-07-25 20:26 ` Peter Zijlstra
2020-07-26 4:14 ` Nicholas Piggin
2020-07-26 11:59 ` peterz
2020-07-26 12:11 ` peterz [this message]
2020-07-28 11:22 ` Nicholas Piggin
2020-08-07 11:11 ` peterz
2020-08-12 8:18 ` Nicholas Piggin
2020-08-12 10:35 ` peterz
2020-08-18 7:22 ` Nicholas Piggin
2020-08-18 15:41 ` peterz
2020-08-18 23:54 ` Nicholas Piggin
2020-08-19 10:39 ` Alexey Kardashevskiy
2020-08-19 15:32 ` peterz
2020-08-19 15:39 ` peterz
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