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

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Message-ID: <20120616132553.GF2231@yliu-dev.sh.intel.com>
Date:	Sat, 16 Jun 2012 21:25:53 +0800
From:	Yuanhan Liu <yuanhan.liu@...ux.intel.com>
To:	Kay Sievers <kay@...y.org>
Cc:	linux-kernel@...r.kernel.org, wfg@...ux.intel.com,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 1/2] printk: use logbuf_mutex_lock to stop syslog_seq
 from going wild

On Sat, Jun 16, 2012 at 03:03:37PM +0200, Kay Sievers wrote:
> On Sat, Jun 16, 2012 at 2:59 PM, Yuanhan Liu
> <yuanhan.liu@...ux.intel.com> 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.
> >
> >> Something like this?
> >
> > BTW, since you already made a patch, should I write a version 2 based
> > on your comments?
> 
> Sure, please do.
> 
> Thanks a lot for doing this,

My pleasure. An updated patch just sent out. Thanks for your comments
very much.

--
yliu
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