linux-kernel - Re: [PATCH 2/4] printk: disable optimistic spin during panic

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Date:   Thu, 27 Jan 2022 13:49:44 +0106
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     Stephen Brennan <stephen.s.brennan@...cle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] printk: disable optimistic spin during panic

On 2022-01-27, Petr Mladek <pmladek@...e.com> wrote:
> I mean that chance of dealock caused by the internal semaohore spin
> lock is super small. In compare, a lot of tricky code is guarded
> by console_sem. It looks like a big risk to ignore the semaphore
> early in panic().

Agreed.

> A better solution would be to use raw_spin_trylock_irqsave() in
> down_trylock().

down_trylock() is attempting to decrement a semaphore. It should not
fail just because another CPU is also in the process of
decrementing/incrementing the semaphore.

Maybe a down_trylock_cond() could be introduced where the trylock could
fail if a given condition is not met. The function would need to
implement its own internal trylock spin loop to check the condition. But
then we could pass in a condition for it to abort. For example, when in
panic and we are not the panic CPU.

John