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

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Message-ID: <20201130130315.GJ3092@hirez.programming.kicks-ass.net>
Date:   Mon, 30 Nov 2020 14:03:15 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sven Schnelle <svens@...ux.ibm.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        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 Mon, Nov 30, 2020 at 01:52:11PM +0100, Peter Zijlstra wrote:
> On Mon, Nov 30, 2020 at 01:31:33PM +0100, Sven Schnelle wrote:
> > [    0.670280] ------------[ cut here ]------------ 
> > [    0.670288] WARNING: CPU: 1 PID: 0 at kernel/rcu/tree.c:1054 rcu_irq_enter+0x7e/0xa8 
> > [    0.670293] Modules linked in: 
> > [    0.670299] CPU: 1 PID: 0 Comm: swapper/1 Tainted: G        W         5.10.0-rc6 #2263 
> > [    0.670304] Hardware name: IBM 2964 NC9 702 (z/VM 6.4.0) 
> > [    0.670309] Krnl PSW : 0404d00180000000 0000000000d8a8da (rcu_irq_enter+0x82/0xa8) 
> > [    0.670318]            R:0 T:1 IO:0 EX:0 Key:0 M:1 W:0 P:0 AS:3 CC:1 PM:0 RI:0 EA:3 
> > [    0.670325] Krnl GPRS: 0000000000000000 0000000080000002 0000000000000001 000000000101fcee 
> > [    0.670331]            0000000000000000 0000000000000000 0000000000000000 0000000000000000 
> > [    0.670337]            000003e00029ff48 0000000000000000 00000000017212d8 0000000000000001 
> > [    0.670343]            0000000005ba0100 00000000000324bb 000003e00029fe40 000003e00029fe10
> >  
> > [    0.670358] Krnl Code: 0000000000d8a8ca: ec180013017e        cij     %r1,1,8,0000000000d8a8f0 
> > [    0.670358]            0000000000d8a8d0: ecb80005007e        cij     %r11,0,8,0000000000d8a8da 
> > [    0.670358]           #0000000000d8a8d6: af000000            mc      0,0 
> > [    0.670358]           >0000000000d8a8da: ebbff0a00004        lmg     %r11,%r15,160(%r15) 
> > [    0.670358]            0000000000d8a8e0: c0f4ffffff68        brcl    15,0000000000d8a7b0 
> > [    0.670358]            0000000000d8a8e6: c0e5000038c1        brasl   %r14,0000000000d91a68 
> > [    0.670358]            0000000000d8a8ec: a7f4ffdc            brc     15,0000000000d8a8a4 
> > [    0.670358]            0000000000d8a8f0: c0e5000038bc        brasl   %r14,0000000000d91a68 
> > [    0.670392] Call Trace: 
> > [    0.670396]  [<0000000000d8a8da>] rcu_irq_enter+0x82/0xa8  
> > [    0.670401]  [<0000000000157f9a>] irq_enter+0x22/0x30  
> > [    0.670404]  [<000000000010e51c>] do_IRQ+0x64/0xd0  
> > [    0.670408]  [<0000000000d9a65a>] ext_int_handler+0x18e/0x194  
> > [    0.670412]  [<0000000000d9a6a0>] psw_idle+0x40/0x48  
> > [    0.670416] ([<0000000000104202>] enabled_wait+0x22/0xf0) 
> > [    0.670419]  [<00000000001046e2>] arch_cpu_idle+0x22/0x38  
> > [    0.670423]  [<0000000000d986cc>] default_idle_call+0x74/0xd8  
> > [    0.670427]  [<000000000019a94a>] do_idle+0xf2/0x1b0  
> > [    0.670431]  [<000000000019ac7e>] cpu_startup_entry+0x36/0x40  
> > [    0.670435]  [<0000000000118b9a>] smp_start_secondary+0x82/0x88  
> 
> 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?