Subject: Re: [RFC PATCH powerpc] Fix a lazy irq related WARING in arch_local_irq_restore()

From: Li Zhong <[email-masked]>
Date: Mon, 24 Sep 2012 14:42:42 +0800

On Mon, 2012-09-24 at 15:30 +1000, Benjamin Herrenschmidt wrote:
> On Mon, 2012-09-24 at 11:56 +0800, Li Zhong wrote:
> > This patch tries to fix a WARNING in irq.c(below), when doing cpu offline/online. 
> > 
> > The reason is that if the preferred offline state is CPU_STATE_INACTIVE, when
> > cpu offline, pseries_mach_cpu_die() calls extended_cede_processor() to cede
> > the processor. After the hv call returns, the MSR_EE is enabled, while
> > the irq_happened in paca should already be set as PACA_IRQ_HARD_DIS.
> > 
> > Then when the cpu is put online again, the warning is reported when
> > start_secondary() tries to enable irq. [ Sometimes, we don't see this warning,
> > that's because when we come to local_irq_enable(), there might already have been
> > some interrupts occurred, e.g. PACA_IRQ_DEC is already set. ]
> > 
> > The patch tries to clear MSR_EE by calling __hard_irq_disable() after cede
> > returns, and before calling start_secondary_resume(). 
> 
Hi Ben,

> Is that enough ? Do we know for sure we won't get a stray IPI or
> interrupt which then will reach us with an inconsistent HW/SW enable
> state ?

I thought when coming to here, there should be no IPI or interrupt for
this cpu ...

> We might need to "sanitize" the enable state in the PACA before we
> actually enter NAP or in the return from NAP code, like we do for normal
> idle code...

Do you mean something like this in extended_cede_processor() to make
sure HW/SW enable state consistent? 

+	prep_irq_for_idle();
        rc = cede_processor();
+#ifdef CONFIG_TRACE_IRQFLAGS
+                /* Ensure that H_CEDE returns with IRQs on */
+                if (WARN_ON(!(mfmsr() & MSR_EE)))
+                        __hard_irq_enable();
+#endif  

So, normally, we will have irqs SW/HW enabled consistently. But if the
above happens (some missing IPI/interrupt ), I'm not sure whether we can
leave it to be handled after cpu online again? 

Please correct me if I misunderstood something. 

Thanks, Zhong

> Cheers,
> Ben.
> 
> > [   56.618846] WARNING: at arch/powerpc/kernel/irq.c:240
> > [   56.618851] Modules linked in: rcutorture ipv6 dm_mod ext3 jbd mbcache sg sd_mod crc_t10dif ibmvscsic scsi_transport_srp scsi_tgt ibmveth
> > [   56.618883] NIP: c00000000000ff94 LR: c00000000067a5e0 CTR: 0000000000000001
> > [   56.618889] REGS: c0000001fef6bbe0 TRAP: 0700   Not tainted  (3.6.0-rc1-autokern1)
> > [   56.618894] MSR: 8000000000029032 <SF,EE,ME,IR,DR,RI>  CR: 42000082  XER: 20000000
> > [   56.618913] SOFTE: 1
> > [   56.618916] CFAR: c00000000067a5dc
> > [   56.618920] TASK = c0000001feed79a0[0] 'swapper/5' THREAD: c0000001fef68000 CPU: 5
> > GPR00: 0000000000000001 c0000001fef6be60 c000000000f9ca08 0000000000000001 
> > GPR04: 0000000000000001 0000000000000008 0000000000000001 0000000000000000 
> > GPR08: 0000000000000000 c0000001feed79a0 0008a80000000000 0000000000000000 
> > GPR12: 0000000022000082 c00000000f330f00 c0000001fef6bf90 000000000f394b4c 
> > GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000 
> > GPR20: 0000000000000000 0000000000000000 0000000000000000 0000000000000000 
> > GPR24: c000000000fe8f80 0000000000000008 0000000000000028 0000000000000000 
> > GPR28: 0000000000000000 0000000000000020 c000000000f1ab40 0000000000000001 
> > [   56.619014] NIP [c00000000000ff94] .arch_local_irq_restore+0x34/0xa0
> > [   56.619020] LR [c00000000067a5e0] .start_secondary+0x368/0x37c
> > [   56.619025] Call Trace:
> > [   56.619030] [c0000001fef6be60] [c000000001ba0500] 0xc000000001ba0500 (unreliable)
> > [   56.619038] [c0000001fef6bed0] [c00000000067a5e0] .start_secondary+0x368/0x37c
> > [   56.619046] [c0000001fef6bf90] [c000000000009380] .start_secondary_resume+0x10/0x14
> > [   56.619052] Instruction dump:
> > [   56.619056] f8010010 f821ff91 986d022a 2fa30000 419e0054 880d022b 78000621 41820048 
> > [   56.619071] 2f800001 40de0064 7c0000a6 78008fe2 <0b000000> 2fa00000 40de0050 38000000 
> > [   56.619088] ---[ end trace 0199c0d783d7f9ba ]---
> > 
> > Reported-by: Paul E. McKenney <[email-masked]>
> > Signed-off-by: Li Zhong <[email-masked]>
> > ---
> >  arch/powerpc/platforms/pseries/hotplug-cpu.c |    2 ++
> >  1 files changed, 2 insertions(+), 0 deletions(-)
> > 
> > diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c
> > index 64c97d8..8de539a 100644
> > --- a/arch/powerpc/platforms/pseries/hotplug-cpu.c
> > +++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c
> > @@ -137,6 +137,8 @@ static void pseries_mach_cpu_die(void)
> >  		if (get_preferred_offline_state(cpu) == CPU_STATE_ONLINE) {
> >  			unregister_slb_shadow(hwcpu);
> >  
> > +			__hard_irq_disable();
> > +
> >  			/*
> >  			 * Call to start_secondary_resume() will not return.
> >  			 * Kernel stack will be reset and start_secondary()
> 
>