linux-kernel - Re: [RFC V2 12/21] rv/reactor: Add the printk reactor

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Date:   Tue, 15 Feb 2022 13:43:35 +0100
From:   Daniel Bristot de Oliveira <bristot@...nel.org>
To:     John Ogness <john.ogness@...utronix.de>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     Jonathan Corbet <corbet@....net>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Marco Elver <elver@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Gabriele Paoloni <gpaoloni@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Clark Williams <williams@...hat.com>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-trace-devel@...r.kernel.org
Subject: Re: [RFC V2 12/21] rv/reactor: Add the printk reactor

On 2/15/22 11:03, John Ogness wrote:
> On 2022-02-14, Daniel Bristot de Oliveira <bristot@...nel.org> wrote:
>> On 2/14/22 18:25, Shuah Khan wrote:
>>> On 2/14/22 3:45 AM, Daniel Bristot de Oliveira wrote:
>>>> Note: do not use this reactor with rq_lock taken, it will lock the
>>>> system until printk can handle that.
>>>
>>> Please give more details on "lock the system" and how locking
>>> would impact users and system operation
>>
>> It is a deadlock on the rq_lock. Hopefully the new printk
>> implementation handles that better.
> 
> I suggest using printk_deferred() for this reactor for now. With
> printk_deferred() the message and timestamp are immediately and
> locklessly stored in the buffer, and the printing is performed in a
> separate context. printk_deferred() is safe for all contexts.

Question: Does it always postpone or only postpone when in a particular
contexts, like, with irqs disabled?

-- Daniel

> John