linux-kernel - Re: [BUG] Something goes wrong with timer statistics.

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Message-Id: <20070530144449.36811a4d.dada1@cosmosbay.com>
Date:	Wed, 30 May 2007 14:44:49 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Björn Steinbrink <B.Steinbrink@....de>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Michal Piotrowski <michal.k.k.piotrowski@...il.com>,
	Ian Kumlien <iank@...dband.net>, Linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [BUG] Something goes wrong with timer statistics.

On Wed, 30 May 2007 13:38:25 +0200
Björn Steinbrink <B.Steinbrink@....de> wrote:

> On 2007.05.30 00:38:08 +0200, Thomas Gleixner wrote:
> > On Wed, 2007-05-30 at 00:24 +0200, Michal Piotrowski wrote:
> > > Hi Ian,
> > > 
> > > (Thomas "The Wizard of Time" added to CC :))
> > 
> > Added more wizards :)
> > 
> > > On 29/05/07, Ian Kumlien <iank@...dband.net> wrote:
> > > > Hi,
> > > >
> > > > As the daystar sets, i try to play some with my new would be
> > > > firewall/server, but since this will be running for quite some time i
> > > > have been experimenting with powertop to find out what i can do to limit
> > > > it's power usage.
> > > >
> > > > But, if i run powertop for too long or a few times to many... this
> > > > happens:
> > > > http://pomac.netswarm.net/pics/kernel_panic.jpg
> > 
> > This was reported before. It's incredibly hard to reproduce.
> 
> OK, second try, this time with a patch. In timer_stats_update_stats,
> input is allocated on the stack, so it is uninitialized, in particular
> the "next" field is random. Now in tstat_lookup, the new entry "curr" is
> initialized with the values from "input" (passed as "entry") and "next"
> is set to NULL _after_ it is added to the list, so if a second CPU is
> running the fastpath lookup while we're inserting the new element, it
> might get the garbage value in "next". The patch below fixes that.
> 
> Björn
> 
> 
> 
> Initialize the "next" field of a timer stats entry before it is inserted
> into the list to avoid a race with the fastpath lookup.
> 
> Signed-off-by: Björn Steinbrink <B.Steinbrink@....de>
> ---
> diff --git a/kernel/time/timer_stats.c b/kernel/time/timer_stats.c
> index 868f1bc..5bc8f91 100644
> --- a/kernel/time/timer_stats.c
> +++ b/kernel/time/timer_stats.c
> @@ -202,12 +202,12 @@ static struct entry *tstat_lookup(struct entry *entry, char *comm)
>  	if (curr) {
>  		*curr = *entry;
>  		curr->count = 0;
> +		curr->next = NULL;
>  		memcpy(curr->comm, comm, TASK_COMM_LEN);
>  		if (prev)
>  			prev->next = curr;
>  		else
>  			*head = curr;
> -		curr->next = NULL;
>  	}
>   out_unlock:
>  	spin_unlock(&table_lock);
> 

Your analysis might be right, not the fix.

You *cannot* assume curr->next = NULL will be done before insert.

You probably also need a memory barrier.

	curr->next = NULL;
	memcpy(curr->comm, comm, TASK_COMM_LEN);

	smp_mb(); /* commit all writes to curr before insert */

	if (prev)
		prev->next = curr;
	else
		*head = curr;
-
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