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Sender: Guenter Roeck <groeck7@gmail.com>
Date:   Wed, 7 Jul 2021 12:18:06 -0700
From:   Guenter Roeck <linux@roeck-us.net>
To:     Valentin Schneider <valentin.schneider@arm.com>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        Bharata B Rao <bharata@linux.ibm.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Subject: Re: [PATCH] powerpc: preempt: Don't touch the idle task's
 preempt_count during hotplug
Message-ID: <20210707191806.GA2205122@roeck-us.net>
MIME-Version: 1.0
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On Wed, Jul 07, 2021 at 07:38:31PM +0100, Valentin Schneider wrote:
> Powerpc currently resets a CPU's idle task preempt_count to 0 before said
> task starts executing the secondary startup routine (and becomes an idle
> task proper).
> 
> This conflicts with commit
> 
>   f1a0a376ca0c ("sched/core: Initialize the idle task with preemption disabled")
> 
> which initializes all of the idle tasks' preempt_count to PREEMPT_DISABLED
> during smp_init(). Note that this was superfluous before said commit, as
> back then the hotplug machinery would invoke init_idle() via
> idle_thread_get(), which would have already reset the CPU's idle task's
> preempt_count to PREEMPT_ENABLED.
> 
> Get rid of this preempt_count write.
> 
> Cc: Guenter Roeck <linux@roeck-us.net>
> Fixes: f1a0a376ca0c ("sched/core: Initialize the idle task with preemption disabled")
> Reported-by: Bharata B Rao <bharata@linux.ibm.com>
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>

Tested-by: Guenter Roeck <linux@roeck-us.net>

> ---
>  arch/powerpc/platforms/cell/smp.c    | 3 ---
>  arch/powerpc/platforms/pseries/smp.c | 5 +----
>  2 files changed, 1 insertion(+), 7 deletions(-)
> 
> diff --git a/arch/powerpc/platforms/cell/smp.c b/arch/powerpc/platforms/cell/smp.c
> index c855a0aeb49c..d7ab868aab54 100644
> --- a/arch/powerpc/platforms/cell/smp.c
> +++ b/arch/powerpc/platforms/cell/smp.c
> @@ -78,9 +78,6 @@ static inline int smp_startup_cpu(unsigned int lcpu)
>  
>  	pcpu = get_hard_smp_processor_id(lcpu);
>  
> -	/* Fixup atomic count: it exited inside IRQ handler. */
> -	task_thread_info(paca_ptrs[lcpu]->__current)->preempt_count	= 0;
> -
>  	/*
>  	 * If the RTAS start-cpu token does not exist then presume the
>  	 * cpu is already spinning.
> diff --git a/arch/powerpc/platforms/pseries/smp.c b/arch/powerpc/platforms/pseries/smp.c
> index 096629f54576..7ebf3382816a 100644
> --- a/arch/powerpc/platforms/pseries/smp.c
> +++ b/arch/powerpc/platforms/pseries/smp.c
> @@ -105,10 +105,7 @@ static inline int smp_startup_cpu(unsigned int lcpu)
>  		return 1;
>  	}
>  
> -	/* Fixup atomic count: it exited inside IRQ handler. */
> -	task_thread_info(paca_ptrs[lcpu]->__current)->preempt_count	= 0;
> -
> -	/* 
> +	/*
>  	 * If the RTAS start-cpu token does not exist then presume the
>  	 * cpu is already spinning.
>  	 */
> -- 
> 2.25.1
>