linux-kernel - Re: [GIT pull] locking/urgent for v5.10-rc6

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Date:   Tue, 1 Dec 2020 06:46:44 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Christian Borntraeger <borntraeger@...ibm.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [GIT pull] locking/urgent for v5.10-rc6

On Tue, Dec 01, 2020 at 12:07:24PM +0100, Peter Zijlstra wrote:
> On Tue, Dec 01, 2020 at 09:07:34AM +0100, Peter Zijlstra wrote:
> > On Mon, Nov 30, 2020 at 08:31:32PM +0100, Christian Borntraeger wrote:
> > > On 30.11.20 19:04, Linus Torvalds wrote:
> > > > On Mon, Nov 30, 2020 at 5:03 AM Peter Zijlstra <peterz@...radead.org> wrote:
> > > >>
> > > >>> But but but...
> > > >>>
> > > >>>   do_idle()                   # IRQs on
> > > >>>     local_irq_disable();      # IRQs off
> > > >>>     defaul_idle_call()        # IRQs off
> > > >>         lockdep_hardirqs_on();  # IRQs off, but lockdep things they're on
> > > >>>       arch_cpu_idle()         # IRQs off
> > > >>>         enabled_wait()        # IRQs off
> > > >>>         raw_local_save()      # still off
> > > >>>         psw_idle()            # very much off
> > > >>>           ext_int_handler     # get an interrupt ?!?!
> > > >>               rcu_irq_enter()   # lockdep thinks IRQs are on <- FAIL
> > > >>
> > > >> I can't much read s390 assembler, but ext_int_handler() has a
> > > >> TRACE_IRQS_OFF, which would be sufficient to re-align the lockdep state
> > > >> with the actual state, but there's some condition before it, what's that
> > > >> test and is that right?
> > > > 
> > > > I think that "psw_idle()" enables interrupts, exactly like x86 does.
> > 
> > (like ye olde x86, modern x86 idles with interrupts disabled)
> > 
> > > Yes, by definition.  Otherwise it would be an software error state.
> > > The interesting part is the lpswe instruction at the end (load PSW) 
> > > which loads the full PSW, which contains interrupt enablement, wait bit,
> > > condition code, paging enablement, machine check enablement the address
> > > and others. The idle psw is enabled for interrupts and has the wait bit
> > > set. If the wait bit is set and interrupts are off this is called "disabled
> > > wait" and is used for panic, shutdown etc. 
> > 
> > OK, but at that point, hardware interrupt state is on, lockdep thinks
> > it's on. And we take an interrupt, just like any old regular interrupt
> > enabled region.
> > 
> > But then the exception handler (ext_int_handler), which I'm assuming is
> > ran by the hardware with hardware interrupts disabled again, should be
> > calling into lockdep to tell interrupts were disabled. IOW that
> > TRACE_IRQS_OFF bit in there.
> > 
> > But that doesn't seem to be working right. Why? Because afaict this is
> > then the exact normal flow of things, but it's only going sideways
> > during this idle thing.
> > 
> > What's going 'funny' ?
> 
> So after having talked to Sven a bit, the thing that is happening, is
> that this is the one place where we take interrupts with RCU being
> disabled. Normally RCU is watching and all is well, except during idle.

Isn't interrupt entry supposed to invoke rcu_irq_enter() at some point?
Or did this fall victim to recent optimizations?

							Thanx, Paul