linux-kernel - Re: [PATCH 2/4] watchdog/softlockup: Use printk_cpu_sync_get

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Date: Tue, 6 Feb 2024 11:21:06 +0100
From: Petr Mladek <pmladek@...e.com>
To: John Ogness <john.ogness@...utronix.de>
Cc: Douglas Anderson <dianders@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Li Zhe <lizhe.67@...edance.com>, Pingfan Liu <kernelfans@...il.com>,
	Lecopzer Chen <lecopzer.chen@...iatek.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] watchdog/softlockup: Use
 printk_cpu_sync_get_irqsave() to serialize reporting

On Fri 2023-12-22 10:36:37, John Ogness wrote:
> On 2023-12-20, Douglas Anderson <dianders@...omium.org> wrote:
> > Instead of introducing a spinlock, use printk_cpu_sync_get_irqsave()
> > and printk_cpu_sync_put_irqrestore() to serialize softlockup
> > reporting. Alone this doesn't have any real advantage over the
> > spinlock, but this will allow us to use the same function in a future
> > change to also serialize hardlockup crawls.
> 
> Thanks for this change. For me, this is the preferred workaround to
> best-effort serialize a particular type of output.

I agree.

The good thing is that dump_stack_lvl() and nmi_cpu_backtrace()
use this lock on its known. Also nmi_trigger_cpumask_backtrace()
prevents parallel calls. It means that the particular backtraces
should be serialized for most callers.

> Hopefully one day we
> will get to implementing printk contexts [0] [1] so that message blocks
> can be inserted atomically.

I didn't think about this possibility. You are right. It might be even
better than the printk_cpu_sync_put_irqrestore() because it allows
passing the lock to a higher priority context and
supports timeout.


Best Regards,
Petr