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

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Date:	Sat, 16 Jun 2012 15:03:37 +0200
From:	Kay Sievers <kay@...y.org>
To:	Yuanhan Liu <yuanhan.liu@...ux.intel.com>
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 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,
Kay
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