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Subject: Re: [PATCH v6 1/3] printk: Add line-buffered printk() API.
References: <1541165517-3557-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp>
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From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Message-ID: <c1f9b048-e4ed-bc69-9ee6-6fe24233de4a@i-love.sakura.ne.jp>
Date: Thu, 8 Nov 2018 20:46:28 +0900
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To: Petr Mladek <pmladek@suse.com>, Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>, Dmitriy Vyukov <dvyukov@google.com>, Steven Rostedt <rostedt@goodmis.org>, Alexander Potapenko <glider@google.com>, Fengguang Wu <fengguang.wu@intel.com>, Josh Poimboeuf <jpoimboe@redhat.com>, LKML <linux-kernel@vger.kernel.org>, Linus Torvalds <torvalds@linux-foundation.org>, Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org, Ingo Molnar <mingo@redhat.com>, Peter Zijlstra <peterz@infradead.org>, Will Deacon <will.deacon@arm.com>

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.