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Date:   Tue, 15 Feb 2022 13:43:35 +0100
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Subject: Re: [RFC V2 12/21] rv/reactor: Add the printk reactor
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To:     John Ogness <john.ogness@linutronix.de>,
        Shuah Khan <skhan@linuxfoundation.org>,
        Steven Rostedt <rostedt@goodmis.org>
Cc:     Jonathan Corbet <corbet@lwn.net>, Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Marco Elver <elver@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Gabriele Paoloni <gpaoloni@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Clark Williams <williams@redhat.com>,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-trace-devel@vger.kernel.org
References: <cover.1644830251.git.bristot@kernel.org>
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 <87v8xg30qc.fsf@jogness.linutronix.de>
From:   Daniel Bristot de Oliveira <bristot@kernel.org>
In-Reply-To: <87v8xg30qc.fsf@jogness.linutronix.de>
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On 2/15/22 11:03, John Ogness wrote:
> On 2022-02-14, Daniel Bristot de Oliveira <bristot@kernel.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