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From:   John Ogness <john.ogness@linutronix.de>
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To:     Sergey Senozhatsky <senozhatsky@chromium.org>
Cc:     Petr Mladek <pmladek@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        Dmitry Safonov <0x7f454c46@gmail.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Stephen Boyd <swboyd@chromium.org>,
        Alexander Potapenko <glider@google.com>,
        "Paul E. McKenney" <paulmck@kernel.org>
Subject: Re: [PATCH next v1 1/2] dump_stack: move cpu lock to printk.c
In-Reply-To: <YLWhlZy8jQR3zpun@google.com>
References: <20210531162051.2325-1-john.ogness@linutronix.de> <20210531162051.2325-2-john.ogness@linutronix.de> <YLWhlZy8jQR3zpun@google.com>
Date:   Tue, 01 Jun 2021 08:58:54 +0200
Message-ID: <87y2bu2hqp.fsf@jogness.linutronix.de>
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On 2021-06-01, Sergey Senozhatsky <senozhatsky@chromium.org> wrote:
> On (21/05/31 18:20), John Ogness wrote:
>> +void printk_cpu_lock(unsigned int *cpu_store, unsigned long *flags)
>> +{
>> +	unsigned int cpu;
>> +
>> +	for (;;) {
>> +		cpu = get_cpu();
>> +
>> +		*cpu_store = atomic_read(&printk_cpulock_owner);
>> +
>> +		if (*cpu_store == -1) {
>> +			local_irq_save(*flags);
>
> Is there any particular reason this does
>
> 	preempt_disable();
> 	cpu = smp_processor_id();
> 	local_irq_safe();
>
> instead of
>
> 	local_irq_safe();
> 	cpu = raw_smp_processor_id();
>
> ?

If the lock is owned by another CPU, there is no need to disable
interrupts for this CPU. (The local_irq_save() is conditional.)

John Ogness