linux-kernel - Re: [PATCH next v1 1/2] dump_stack: move cpu lock to printk.c

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Message-ID: <87y2bu2hqp.fsf@jogness.linutronix.de>
Date:   Tue, 01 Jun 2021 08:58:54 +0200
From:   John Ogness <john.ogness@...utronix.de>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Alexander Potapenko <glider@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH next v1 1/2] dump_stack: move cpu lock to printk.c

On 2021-06-01, Sergey Senozhatsky <senozhatsky@...omium.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