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Date: Mon, 23 Jul 2007 14:05:41 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: "Mike Frysinger" <vapier.adi@gmail.com>
Cc: "Robin Getz" <rgetz@blackfin.uclinux.org>, linux-kernel@vger.kernel.org,
       "Greg Ungerer" <gerg@snapgear.com>,
       "Russell King" <rmk@arm.linux.org.uk>,
       "Paul Mundt" <lethal@linux-sh.org>, "Tim Bird" <tim.bird@am.sony.com>,
       bryan.wu@analog.com
Subject: Re: early_printk accessing __log_buf
Message-Id: <20070723140541.8fac78c5.akpm@linux-foundation.org>
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On Mon, 23 Jul 2007 16:54:36 -0400
"Mike Frysinger" <vapier.adi@gmail.com> wrote:

> On 7/23/07, Andrew Morton <akpm@linux-foundation.org> wrote:
> > On Mon, 23 Jul 2007 14:19:12 -0400
> > Robin Getz <rgetz@blackfin.uclinux.org> wrote:
> >
> > > On Sun 22 Jul 2007 19:50, Mike Frysinger pondered:
> > > >
> > > > i think the attached two functions account for what Robin and Andrew
> > > > were thinking ...
> > >
> > > A note about why/when grab_lock would be set to zero (pre-kernel init, or
> > > OOPs) might be nice.
> >
> > Definitely.
> >
> > The pre-kernel init shouldn't be an issue: logbuf_lock is initialised at
> > compile time.
> >
> > At oops time we could possibly use oops_in_progress to work out whether to
> > avoid taking the lock.  That's not terribly nice, but nor is it nice for
> > callers to know about printk internals.
> 
> maybe, but for early debug users (the reason we wanted this
> originally), it wouldnt be an oops in progress ... but i guess we can
> just as easily set oops_in_progress to 1 in our code before calling
> this function to keep from having to worry over locks from being
> doubly grabbed.

I don't immediately see how logbuf_lock could be doubly grabbed.  Only if
you're calling this from hard irq context?