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

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Date:   Thu, 3 Jun 2021 08:33:06 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.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 Tue 2021-06-01 16:21:52, John Ogness wrote:
> On 2021-06-01, Petr Mladek <pmladek@...e.com> wrote:
> >> +void printk_cpu_lock(unsigned int *cpu_store, unsigned long *flags)
> >
> > I think about calling this printk_cpu_lock_irqsave() to make it clear
> > that it disables interrupts.
> 
> Agreed.
> 
> > Strictly speaking, it should be enough to disable preemption. If it is
> > safe when interrupted by NMI, it must be safe also when interrupted
> > by a normal interrupt.
> >
> > I guess that the interrupts are disabled because it reduces the risk
> > of nested (messed) backtraces.
> 
> If it was just about synchronizing output triggered by sysreq, then it
> probably would be acceptable to leave interrupts active. But when atomic
> consoles are involved, we are talking about a crashing machine that is
> trying to get log messages out. Any interrupt is a risk that the machine
> may not survive long enough to return from that interruption.

Fair enough. It might be good to mention this motivation in the commit
message or in a code commentary. IMHO, it is always good to know
whether these things a must to have or if there is another reason.
Anyway, it was not obvious to me ;-)

Best Regards,
Petr