linux-kernel - Re: [PATCH v6 2/2] printk: Make printing of spin

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Message-ID: <20160314041747.GF5220@X58A-UD3R>
Date:	Mon, 14 Mar 2016 13:17:48 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	akpm@...ux-foundation.org, mingo@...nel.org,
	linux-kernel@...r.kernel.org, akinobu.mita@...il.com, jack@...e.cz,
	peter@...leysoftware.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org
Subject: Re: [PATCH v6 2/2] printk: Make printing of spin_dump() deferred to
 avoid a deadlock

On Mon, Mar 14, 2016 at 11:40:55AM +0900, Sergey Senozhatsky wrote:
> Hi,
> 
> On (03/14/16 11:30), Byungchul Park wrote:
> [..]
> > > so can it be
> > > 
> > > vprintk_emit()
> > >  __spin_dump_deferred()
> > >   vprintk_deferred()
> > >    vprintk_emit()
> > >     __spin_dump_deferred()
> >       ^^^
> >       can be caused by raw_spin_lock(logbug_lock)
> > 
> > >      vprintk_deferred()
> > 
> > Yes, it can happen by raw_spin_lock(logbuf_lock) to print warning or error
> > message. Are you worrying about an infinite recursion?
> 
> yes.
> 
> > 1. In the case printing warning, eventually it can fill the buffer without
> > actual printing using console.
> 
> so the worry is that the CPU that spins on __spin_dump_deferred() has IRQs
> disabled and `printk_pending' bit set; but IRQ may never be enabled on this
> CPU.

It's true if another cpu never release the logbug_lock. But it's another
problem which current code already has and must be solved by another way.
IOW, the situation where the logbug_lock is not held yet is not the case
where we need to worry about whether the "IRQ work queue" works or not.

> 
> > 2. In the case printing error, the infinite recursion can be prevented by
> > debug_locks_off().
> > 
> > Therefore, no problem.
> 
> 	-ss