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

[Christophe Leroy <christophe.leroy@c-s.fr>]

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