linux-kernel - Re: [PATCH v6 1/3] printk: Add line-buffered printk() API.

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Date:   Thu, 8 Nov 2018 20:46:28 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Dmitriy Vyukov <dvyukov@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Alexander Potapenko <glider@...gle.com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH v6 1/3] printk: Add line-buffered printk() API.

On 2018/11/08 20:24, Petr Mladek wrote:
>> Let's have one more look at what we will fix and what we will break.
>>
>> 'cont' has premature flushes.
>>
>>   Why is it good.
>>   It preserves the correct order of events.
>>
>>   pr_cont("calling foo->init()....");
>>   foo->init()
>>    printk("Can't allocate buffer\n");    // premature flush
>>   pr_cont("...blah\h");
>>
>>  Will end up in the logbuf as:
>>  [12345.123] calling foo->init()....
>>  [12345.124] Can't allocate buffer
>>  [12345.125] ...blah
>>
>>  Where buffered printk will endup as:
>>  [12345.123] Can't allocate buffer
>>  [12345.124] calling foo->init().......blah
> 
> We will always have this problem with API using explicit buffers.
> What do you suggest instead, please?
> 
> I am afraid that we are running in cycles. The other serious
> alternative was having per-process and per-context buffers
> but it was rejected several times.

Is it possible to identify all locations which should use their own
printk() buffers (e.g. interrupt handlers, oops handlers) ? If yes,
automatically switching printk() buffers (like memalloc_nofs_save()/
memalloc_nofs_restore()) will be easiest and least error prone.