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Subject: Re: [PATCH 1/1] ppc/crash: Skip spinlocks during crash
To:     Leonardo Bras <leonardo@linux.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Enrico Weigelt <info@metux.net>,
        Allison Randal <allison@lohutok.net>,
        Thomas Gleixner <tglx@linutronix.de>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
References: <20200326222836.501404-1-leonardo@linux.ibm.com>
From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <af505ef0-e0df-e0aa-bb83-3ed99841f151@c-s.fr>
Date:   Fri, 27 Mar 2020 07:50:20 +0100
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Le 26/03/2020 à 23:28, Leonardo Bras a écrit :
> During a crash, there is chance that the cpus that handle the NMI IPI
> are holding a spin_lock. If this spin_lock is needed by crashing_cpu it
> will cause a deadlock. (rtas_lock and printk logbuf_log as of today)
> 
> This is a problem if the system has kdump set up, given if it crashes
> for any reason kdump may not be saved for crash analysis.
> 
> Skip spinlocks after NMI IPI is sent to all other cpus.
> 
> Signed-off-by: Leonardo Bras <leonardo@linux.ibm.com>
> ---
>   arch/powerpc/include/asm/spinlock.h | 6 ++++++
>   arch/powerpc/kexec/crash.c          | 3 +++
>   2 files changed, 9 insertions(+)
> 
> diff --git a/arch/powerpc/include/asm/spinlock.h b/arch/powerpc/include/asm/spinlock.h
> index 860228e917dc..a6381d110795 100644
> --- a/arch/powerpc/include/asm/spinlock.h
> +++ b/arch/powerpc/include/asm/spinlock.h
> @@ -111,6 +111,8 @@ static inline void splpar_spin_yield(arch_spinlock_t *lock) {};
>   static inline void splpar_rw_yield(arch_rwlock_t *lock) {};
>   #endif
>   
> +extern bool crash_skip_spinlock __read_mostly;
> +
>   static inline bool is_shared_processor(void)
>   {
>   #ifdef CONFIG_PPC_SPLPAR
> @@ -142,6 +144,8 @@ static inline void arch_spin_lock(arch_spinlock_t *lock)
>   		if (likely(__arch_spin_trylock(lock) == 0))
>   			break;
>   		do {
> +			if (unlikely(crash_skip_spinlock))
> +				return;

You are adding a test that reads a global var in the middle of a so hot 
path ? That must kill performance. Can we do different ?

Christophe