linux-kernel - Re: [PATCH 1/1] ppc/crash: Skip spinlocks during crash

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Date:   Thu, 26 Mar 2020 20:26:59 -0300
From:   Leonardo Bras <leonardo@...ux.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Enrico Weigelt <info@...ux.net>,
        Allison Randal <allison@...utok.net>,
        Christophe Leroy <christophe.leroy@....fr>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 1/1] ppc/crash: Skip spinlocks during crash

oops, forgot to EXPORT_SYMBOL. 
arch_spin_lock*() is used on modules.

Sending v2.

On Thu, 2020-03-26 at 19:28 -0300, Leonardo Bras wrote:
> 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@...ux.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;
>  			HMT_low();
>  			if (is_shared_processor())
>  				splpar_spin_yield(lock);
> @@ -161,6 +165,8 @@ void arch_spin_lock_flags(arch_spinlock_t *lock, unsigned long flags)
>  		local_save_flags(flags_dis);
>  		local_irq_restore(flags);
>  		do {
> +			if (unlikely(crash_skip_spinlock))
> +				return;
>  			HMT_low();
>  			if (is_shared_processor())
>  				splpar_spin_yield(lock);
> diff --git a/arch/powerpc/kexec/crash.c b/arch/powerpc/kexec/crash.c
> index d488311efab1..8a522380027d 100644
> --- a/arch/powerpc/kexec/crash.c
> +++ b/arch/powerpc/kexec/crash.c
> @@ -66,6 +66,8 @@ static int handle_fault(struct pt_regs *regs)
>  
>  #ifdef CONFIG_SMP
>  
> +bool crash_skip_spinlock;
> +
>  static atomic_t cpus_in_crash;
>  void crash_ipi_callback(struct pt_regs *regs)
>  {
> @@ -129,6 +131,7 @@ static void crash_kexec_prepare_cpus(int cpu)
>  	/* Would it be better to replace the trap vector here? */
>  
>  	if (atomic_read(&cpus_in_crash) >= ncpus) {
> +		crash_skip_spinlock = true;
>  		printk(KERN_EMERG "IPI complete\n");
>  		return;
>  	}

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