linux-kernel - Re: [PATCH 1/3] printk: Release console_sem after logbuf

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Message-ID: <1307698844.3941.105.camel@twins>
Date:	Fri, 10 Jun 2011 11:40:44 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, efault@....de,
	Arne Jansen <lists@...-jansens.de>
Subject: Re: [PATCH 1/3] printk: Release console_sem after logbuf_lock

On Fri, 2011-06-10 at 11:33 +0200, Ingo Molnar wrote:
> * Andrew Morton <akpm@...ux-foundation.org> wrote:
> 
> > > Some notes:
> > 
> > Stupid thing doesn't explain the magical locking though :( I'm 
> > 99.9% sure that putting an up() inside a spinlock_irq()ed region 
> > was deliberate.
> 
> My guess would be it's done so that pending irqs that have queued up 
> during our current printk-ing activities do not hit us with the 
> console still locked.

Ah, so we already flushed the buffer, but have console_sem locked, so
any interrupt that comes in and prints something will place it in the
buffer but find console_sem is taken, so not flush it.

Then when we're back to doing up() the buffer is filled and nobody will
flush it.

I guess, we can write it like:

	spin_unlock(&logbuf_lock);
	up(&console_sem);
	local_irq_restore(flags);

which would keep interrupt disabled over up(), but have the logbuf_lock
dropped.

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