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

[Heiko Carstens]

On Wed, Dec 02, 2020 at 11:16:05AM +0000, Mark Rutland wrote:
> On Wed, Dec 02, 2020 at 11:56:49AM +0100, Heiko Carstens wrote:
> > From 7bd86fb3eb039a4163281472ca79b9158e726526 Mon Sep 17 00:00:00 2001
> > From: Heiko Carstens <hca@xxxxxxxxxxxxx>
> > Date: Wed, 2 Dec 2020 11:46:01 +0100
> > Subject: [PATCH] s390: fix irq state tracing
> > 
> > With commit 58c644ba512c ("sched/idle: Fix arch_cpu_idle() vs
> > tracing") common code calls arch_cpu_idle() with a lockdep state that
> > tells irqs are on.
> > 
> > This doesn't work very well for s390: psw_idle() will enable interrupts
> > to wait for an interrupt. As soon as an interrupt occurs the interrupt
> > handler will verify if the old context was psw_idle(). If that is the
> > case the interrupt enablement bits in the old program status word will
> > be cleared.
> > 
> > A subsequent test in both the external as well as the io interrupt
> > handler checks if in the old context interrupts were enabled. Due to
> > the above patching of the old program status word it is assumed the
> > old context had interrupts disabled, and therefore a call to
> > TRACE_IRQS_OFF (aka trace_hardirqs_off_caller) is skipped. Which in
> > turn makes lockdep incorrectly "think" that interrupts are enabled
> > within the interrupt handler.
> > 
> > Fix this by unconditionally calling TRACE_IRQS_OFF when entering
> > interrupt handlers. Also call unconditionally TRACE_IRQS_ON when
> > leaving interrupts handlers.
> > 
> > This leaves the special psw_idle() case, which now returns with
> > interrupts disabled, but has an "irqs on" lockdep state. So callers of
> > psw_idle() must adjust the state on their own, if required. This is
> > currently only __udelay_disabled().
> > 
> > Fixes: 58c644ba512c ("sched/idle: Fix arch_cpu_idle() vs tracing")
> > Signed-off-by: Heiko Carstens <hca@xxxxxxxxxxxxx>
> 
> FWIW, this makes sense to me from what I had to chase on the arm64 side,
> and this seems happy atop v5.10-rc6 with all the lockdep and RCU debug
> options enabled when booting to userspace under QEMU.
> 
> Thanks,
> Mark.

Thanks a lot for having a look and testing this!