linux-kernel - Re: [PATCH 1/2] printk: use logbuf_mutex_lock to stop syslog

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Message-ID: <20120616152836.GB7914@kroah.com>
Date:	Sat, 16 Jun 2012 08:28:36 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Yuanhan Liu <yuanhan.liu@...ux.intel.com>
Cc:	Fengguang Wu <wfg@...ux.intel.com>, Kay Sievers <kay@...y.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] printk: use logbuf_mutex_lock to stop syslog_seq
 from going wild

On Sat, Jun 16, 2012 at 09:28:32PM +0800, Yuanhan Liu wrote:
> On Sat, Jun 16, 2012 at 09:20:52PM +0800, Fengguang Wu wrote:
> > On Sat, Jun 16, 2012 at 08:59:22PM +0800, Yuanhan Liu wrote:
> > > On Sat, Jun 16, 2012 at 02:42:38PM +0200, Kay Sievers wrote:
> > > > On Sat, 2012-06-16 at 12:40 +0800, Yuanhan Liu wrote:
> > > > > Although syslog_seq and log_next_seq stuff are protected by logbuf_lock
> > > > > spin log, it's not enough. Say we have two processes A and B, and let
> > > > > syslog_seq = N, while log_next_seq = N + 1, and the two processes both
> > > > > come to syslog_print at almost the same time. And No matter which
> > > > > process get the spin lock first, it will increase syslog_seq by one,
> > > > > then release spin lock; thus later, another process increase syslog_seq
> > > > > by one again. In this case, syslog_seq is bigger than syslog_next_seq.
> > > > > And latter, it would make:
> > > > >    wait_event_interruptiable(log_wait, syslog != log_next_seq)
> > > > > don't wait any more even there is no new write comes. Thus it introduce
> > > > > a infinite loop reading.
> > > > 
> > > > Oh, multiple readers on the same shared file descriptor are not useful,
> > > > but sure, that needs fixing. Thanks for tracking that down!
> > > > 
> > > > Looks like the same issue existed in the original code already, it's
> > > > just that it was granular at a single character level, and not a line,
> > > > and the seqnum which icreases one-by-one, so the issue was hard to
> > > > trigger.
> > > 
> > > Yes,  I think so, too.
> > > 
> > > > 
> > > > We better make the mutexes interruptible, right?
> > > 
> > > Yes, you are right.
> > 
> > It might be better to do them in two standalone patches?
> > One is a bug fix, the other improves user responsiveness.
> > 
> > Either way, you may add my superficial
> > 
> > Reviewed-by: Fengguang Wu <fengguang.wu@...el.com>
> 
> Too late; I just sent it out ;(
> 
> Anyway, thanks for your Reviewed-by very much. I hope commiter can add
> this for me.

I will do so, thanks.

greg k-h
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