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

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Message-ID: <YfKrcQwtXjR87E3q@alley>
Date:   Thu, 27 Jan 2022 15:25:53 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        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 Thu 2022-01-27 13:49:44, John Ogness wrote:
> 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.

IMHO, it does not matter. As you say, raw_spin_trylock_irqsave() fails
only when another process is about to release or take the semaphore.
The semaphore is usually taken for a long time. The tiny window when
the counter is manipulated is negligible.

I mean, if down_trylock() fails because of raw_spin_trylock_irqsave()
failure then it is few instructions from failing even with the lock.

> 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.

This looks too complicated.

Another solution would be to introduce panic_down_trylock() variant
of down_trylock() that will use raw_spin_trylock_irqsave(). The normal
down_trylock() might still use the raw_spin_lock_irqsave().

Well, this should get discussed with the locking people.

Best Regards,
Petr