Message-Id: <CN6WCMKCWHOG.LT2QV3910UJ2@bobo>
Date:   Tue, 27 Sep 2022 14:25:14 +1000
From:   "Nicholas Piggin" <npiggin@...il.com>
To:     "Zhouyi Zhou" <zhouzhouyi@...il.com>, <mpe@...erman.id.au>,
        <christophe.leroy@...roup.eu>, <atrajeev@...ux.vnet.ibm.com>,
        <linuxppc-dev@...ts.ozlabs.org>, <linux-kernel@...r.kernel.org>,
        <lance@...osl.org>, <paulmck@...nel.org>, <rcu@...r.kernel.org>
Subject: Re: [PATCH linux-next][RFC] powerpc: avoid lockdep when we are
 offline

On Tue Sep 27, 2022 at 11:48 AM AEST, Zhouyi Zhou wrote:
> This is second version of my fix to PPC's  "WARNING: suspicious RCU usage",
> I improved my fix under Paul E. McKenney's guidance:
> Link: https://lore.kernel.org/lkml/20220914021528.15946-1-zhouzhouyi@gmail.com/T/
>
> During the cpu offlining, the sub functions of xive_teardown_cpu will
> call __lock_acquire when CONFIG_LOCKDEP=y. The latter function will
> travel RCU protected list, so "WARNING: suspicious RCU usage" will be
> triggered.
>
> Avoid lockdep when we are offline.

I don't see how this is safe. If RCU is no longer watching the CPU then
the memory it is accessing here could be concurrently freed. I think the
warning is valid.

powerpc's problem is that cpuhp_report_idle_dead() is called before
arch_cpu_idle_dead(), so it must not rely on any RCU protection there.
I would say xive cleanup just needs to be done earlier. I wonder why it
is not done in __cpu_disable or thereabouts, that's where the interrupt
controller is supposed to be stopped.

Thanks,
Nick

>
> Signed-off-by: Zhouyi Zhou <zhouzhouyi@...il.com>
> ---
> Dear PPC and RCU developers
>
> I found this bug when trying to do rcutorture tests in ppc VM of
> Open Source Lab of Oregon State University
>
> console.log report following bug:
> [   37.635545][    T0] WARNING: suspicious RCU usage^M
> [   37.636409][    T0] 6.0.0-rc4-next-20220907-dirty #8 Not tainted^M
> [   37.637575][    T0] -----------------------------^M
> [   37.638306][    T0] kernel/locking/lockdep.c:3723 RCU-list traversed in non-reader section!!^M
> [   37.639651][    T0] ^M
> [   37.639651][    T0] other info that might help us debug this:^M
> [   37.639651][    T0] ^M
> [   37.641381][    T0] ^M
> [   37.641381][    T0] RCU used illegally from offline CPU!^M
> [   37.641381][    T0] rcu_scheduler_active = 2, debug_locks = 1^M
> [   37.667170][    T0] no locks held by swapper/6/0.^M
> [   37.668328][    T0] ^M
> [   37.668328][    T0] stack backtrace:^M
> [   37.669995][    T0] CPU: 6 PID: 0 Comm: swapper/6 Not tainted 6.0.0-rc4-next-20220907-dirty #8^M
> [   37.672777][    T0] Call Trace:^M
> [   37.673729][    T0] [c000000004653920] [c00000000097f9b4] dump_stack_lvl+0x98/0xe0 (unreliable)^M
> [   37.678579][    T0] [c000000004653960] [c0000000001f2eb8] lockdep_rcu_suspicious+0x148/0x16c^M
> [   37.680425][    T0] [c0000000046539f0] [c0000000001ed9b4] __lock_acquire+0x10f4/0x26e0^M
> [   37.682450][    T0] [c000000004653b30] [c0000000001efc2c] lock_acquire+0x12c/0x420^M
> [   37.684113][    T0] [c000000004653c20] [c0000000010d704c] _raw_spin_lock_irqsave+0x6c/0xc0^M
> [   37.686154][    T0] [c000000004653c60] [c0000000000c7b4c] xive_spapr_put_ipi+0xcc/0x150^M
> [   37.687879][    T0] [c000000004653ca0] [c0000000010c72a8] xive_cleanup_cpu_ipi+0xc8/0xf0^M
> [   37.689856][    T0] [c000000004653cf0] [c0000000010c7370] xive_teardown_cpu+0xa0/0xf0^M
> [   37.691877][    T0] [c000000004653d30] [c0000000000fba5c] pseries_cpu_offline_self+0x5c/0x100^M
> [   37.693882][    T0] [c000000004653da0] [c00000000005d2c4] arch_cpu_idle_dead+0x44/0x60^M
> [   37.695739][    T0] [c000000004653dc0] [c0000000001c740c] do_idle+0x16c/0x3d0^M
> [   37.697536][    T0] [c000000004653e70] [c0000000001c7a1c] cpu_startup_entry+0x3c/0x40^M
> [   37.699694][    T0] [c000000004653ea0] [c00000000005ca20] start_secondary+0x6c0/0xb50^M
> [   37.701742][    T0] [c000000004653f90] [c00000000000d054] start_secondary_prolog+0x10/0x14^M
>
>
> Tested on PPC VM of Open Source Lab of Oregon State University.
> Test results show that although "WARNING: suspicious RCU usage" has gone,
> and there are less "BUG: soft lockup" reports than the original kernel
> (9 vs 13), which sounds good ;-)
>
> But after my modification, results-rcutorture-kasan/SRCU-P/console.log.diags
> shows a new warning:
> [  222.289242][  T110] WARNING: CPU: 6 PID: 110 at kernel/rcu/rcutorture.c:2806 rcu_torture_fwd_prog+0xc88/0xdd0
>
> I guess above new warning also exits in original kernel, so I write a tiny test script as follows:
>
> #!/bin/sh
>
> COUNTER=0
> while [ $COUNTER -lt 1000 ] ; do
>     qemu-system-ppc64 -nographic -smp cores=8,threads=1 -net none -M pseries -nodefaults -device spapr-vscsi -serial file:/tmp/console.log -m 2G -kernel /tmp/vmlinux -append "debug_boot_weak_hash panic=-1 console=ttyS0 rcupdate.rcu_cpu_stall_suppress_at_boot=1 torture.disable_onoff_at_boot rcupdate.rcu_task_stall_timeout=30000 rcutorture.torture_type=srcud rcupdate.rcu_self_test=1 rcutorture.fwd_progress=3 srcutree.big_cpu_lim=5 rcutorture.onoff_interval=1000 rcutorture.onoff_holdoff=30 rcutorture.n_barrier_cbs=4 rcutorture.stat_interval=15 rcutorture.shutdown_secs=420 rcutorture.test_no_idle_hz=1 rcutorture.verbose=1"&
>     qemu_pid=$!
>     cd ~/next1/linux-next
>     make clean
> #I use "make vmlinux -j 8" to create heavy background jitter
>     make vmlinux -j 8  > /dev/null 2>&1 
>     make_pid=$!
>     wait $qemu_pid
>     kill $qemu_pid
>     kill $make_id
>     if grep -q WARN /tmp/console.log;
>     then
>         echo $COUNTER > /tmp/counter
>         exit
>     fi
>     COUNTER=$(($COUNTER+1))
> done
>
> Above test shows that original kernel also warn about
> "WARNING: CPU: 6 PID: 110 at kernel/rcu/rcutorture.c:2806 rcu_torture_fwd_prog+0xc88/0xdd0"
>
> But I am not very sure about my results, so I still add a [RFC] to my subject line.
>
> Thank all of you for your guidance and encouragement ;-)
>
> Cheers
> Zhouyi
> --
>  arch/powerpc/platforms/pseries/hotplug-cpu.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c
> index e0a7ac5db15d..e47098f00da1 100644
> --- a/arch/powerpc/platforms/pseries/hotplug-cpu.c
> +++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c
> @@ -64,10 +64,15 @@ static void pseries_cpu_offline_self(void)
>  
>  	local_irq_disable();
>  	idle_task_exit();
> +	/* prevent lockdep code from traveling RCU protected list
> +	 * when we are offline.
> +	 */
> +	lockdep_off();
>  	if (xive_enabled())
>  		xive_teardown_cpu();
>  	else
>  		xics_teardown_cpu();
> +	lockdep_on();
>  
>  	unregister_slb_shadow(hwcpu);
>  	rtas_stop_self();
> -- 
> 2.25.1

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