linux-kernel - Re: possible circular locking dependency detected [was: linux-next: Tree for Aug 22]

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Date:   Thu, 24 Aug 2017 13:49:12 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Byungchul Park <byungchul.park@....com>,
        Bart Van Assche <Bart.VanAssche@....com>,
        peterz@...radead.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "sfr@...b.auug.org.au" <sfr@...b.auug.org.au>,
        "linux-next@...r.kernel.org" <linux-next@...r.kernel.org>,
        kernel-team@....com
Subject: Re: possible circular locking dependency detected [was: linux-next:
 Tree for Aug 22]

Hi,

On (08/24/17 12:39), Boqun Feng wrote:
> On Wed, Aug 23, 2017 at 02:55:17PM +0900, Sergey Senozhatsky wrote:
> > On (08/23/17 13:35), Boqun Feng wrote:
> > > > KERN_CONT and "\n" should not be together. "\n" flushes the cont
> > > > buffer immediately.
> > > > 
> > > 
> > > Hmm.. Not quite familiar with printk() stuffs, but I could see several
> > > usages of printk(KERN_CONT "...\n") in kernel.
> > > 
> > > Did a bit research myself, and I now think the inappropriate use is to
> > > use a KERN_CONT printk *after* another printk ending with a "\n".
> > 
> > ah... I didn't check __print_lock_name(): it leaves unflushed cont buffer
> > upon the return. sorry, your code is correct.
> > 
> 
> So means printk(KERN_CON "..."); + printk(KERN_CONT "...\n") is a
> correct usage, right?

well, yes. with one precondition - there should be no printk-s from other
CPUs/tasks in between

	printk(KERN_CON "...");  +  printk(KERN_CONT "...\n")
                               ^^^^^
			here we can have a preliminary flush and broken
			cont line. but it's been this way forever.

	-ss