linux-kernel - Re: [PATCH RESEND 1/3] printk: convert byte-buffer to variable-length record buffer

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Message-ID: <20120510201409.GA6467@thunk.org>
Date:	Thu, 10 May 2012 16:14:09 -0400
From:	Ted Ts'o <tytso@....edu>
To:	Kay Sievers <kay@...y.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Jonathan Corbet <corbet@....net>,
	Sasha Levin <levinsasha928@...il.com>,
	Greg Kroah-Hartmann <greg@...ah.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND 1/3] printk: convert byte-buffer to
 variable-length record buffer

On Thu, May 10, 2012 at 09:09:46PM +0200, Kay Sievers wrote:
> We fully isolate continuation users from non-continuation users. If a
> continuation user gets interrupted by an ordinary non-continuation
> user, we will not touch the continuation buffer, we just emit the
> ordinary message. When the same thread comes back and continues its
> printing, we still append to the earlier buffer we stored.

It's not necessarily a matter of "thread comes back", although that
situation can happen too.  You can get this situation quite simply if
you have two processes in foreground kernel mode on two different
CPU's sending continuation printk's at the same time.

> We will also never wrongly merge two racing continuation users into one
> line.

I'm not sure how you guarantee this?  The only way you *could*
guarantee this is if you used a continuation buffer in the task_struct
for foreground kernel code, and a per-CPU continuation buffer for
interrupt code.

> Buffered line will be joined, when the same thread emits a printk()
> without any KERN_* or with KERN_CONT.

Is there any difference in any of the cases in terms of how printk's
that are prefixed with KERN_CONT versus a printk that does not have
any KERN_* prefix?  If so, is there value in keeping KERN_CONT?

    	   	       	      	    	     - Ted
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