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Message-ID: <20220914121717.GY246308@paulmck-ThinkPad-P17-Gen-1>
Date: Wed, 14 Sep 2022 05:17:17 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Zhouyi Zhou <zhouzhouyi@...il.com>
Cc: mpe@...erman.id.au, npiggin@...il.com, christophe.leroy@...roup.eu,
atrajeev@...ux.vnet.ibm.com, linuxppc-dev@...ts.ozlabs.org,
linux-kernel@...r.kernel.org, lance@...osl.org, rcu@...r.kernel.org
Subject: Re: [PATCH linux-next][RFC] powerpc: protect cpu offlining by RCU
offline lock
On Wed, Sep 14, 2022 at 10:15:28AM +0800, Zhouyi Zhou wrote:
> 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.
>
> Try to protect cpu offlining by RCU offline lock.
Rather than acquiring the RCU lock, why not change the functions called
by xive_teardown_cpu() to avoid calling __lock_acquire()? For example,
a call to spin_lock() could be changed to arch_spin_lock().
Thanx, Paul
> Tested on PPC VM of Open Source Lab of Oregon State University.
> (Each round of tests takes about 19 hours to finish)
> Test results show that although "WARNING: suspicious RCU usage" has gone,
> but there are more "BUG: soft lockup" reports than the original kernel
> (10 vs 6), so I add a [RFC] to my subject line.
>
> Signed-off-by: Zhouyi Zhou <zhouzhouyi@...il.com>
> ---
> [it seems that there are some delivery problem in my previous email,
> so I send again via gmail, sorry for the trouble]
>
> 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
>
>
> I am a beginner, hope I can be of some beneficial to the community ;-)
>
> Thanks
> Zhouyi
> --
> arch/powerpc/platforms/pseries/hotplug-cpu.c | 5 ++++-
> include/linux/rcupdate.h | 3 ++-
> kernel/rcu/tree.c | 10 ++++++++++
> 3 files changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c
> index 0f8cd8b06432..ddf66a253c70 100644
> --- a/arch/powerpc/platforms/pseries/hotplug-cpu.c
> +++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c
> @@ -64,11 +64,14 @@ static void pseries_cpu_offline_self(void)
>
> local_irq_disable();
> idle_task_exit();
> +
> + /* Because the cpu is now offline, let rcu know that */
> + rcu_state_ofl_lock();
> if (xive_enabled())
> xive_teardown_cpu();
> else
> xics_teardown_cpu();
> -
> + rcu_state_ofl_unlock();
> unregister_slb_shadow(hwcpu);
> rtas_stop_self();
>
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 63d2e6a60ad7..d857955a02ba 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -1034,5 +1034,6 @@ rcu_head_after_call_rcu(struct rcu_head *rhp, rcu_callback_t f)
> /* kernel/ksysfs.c definitions */
> extern int rcu_expedited;
> extern int rcu_normal;
> -
> +void rcu_state_ofl_lock(void);
> +void rcu_state_ofl_unlock(void);
> #endif /* __LINUX_RCUPDATE_H */
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 6bb8e72bc815..3282725f1054 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -4796,6 +4796,16 @@ void __init rcu_init(void)
> (void)start_poll_synchronize_rcu_expedited();
> }
>
> +void rcu_state_ofl_lock(void)
> +{
> + arch_spin_lock(&rcu_state.ofl_lock);
> +}
> +
> +void rcu_state_ofl_unlock(void)
> +{
> + arch_spin_unlock(&rcu_state.ofl_lock);
> +}
> +
> #include "tree_stall.h"
> #include "tree_exp.h"
> #include "tree_nocb.h"
> --
> 2.34.1
>
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