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

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Message-Id: <1180652361.30698.11.camel@pi.pomac.com>
Date:	Fri, 01 Jun 2007 00:59:11 +0200
From:	Ian Kumlien <iank@...dband.net>
To:	Björn Steinbrink <B.Steinbrink@....de>
Cc:	Eric Dumazet <dada1@...mosbay.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Michal Piotrowski <michal.k.k.piotrowski@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [BUG] Something goes wrong with timer statistics.

On tor, 2007-05-31 at 17:27 +0200, Björn Steinbrink wrote:
> On 2007.05.31 17:10:07 +0200, Eric Dumazet wrote:
> > Well... :) , there is still a memory barrier missing it seems.
> > 
> > Another cpu might see a bad value if 'active=1' is set before tstat_hash_table is really cleared.
> 
> Hm, that even makes the assumption in my first try valid ;-)
> Just for the record, this time I thought that the barrier from the
> spinlock in timer_stats_update_stats (right before the check for active)
> would be enough, but that's obviously running on the wrong cpu if we
> race... *sigh*
> 
> Thanks,
> Björn
> 
> 
> 
> Fix two races in the timer stats lookup code. One by ensuring that the
> initialization of a new entry is finished upon insertion of that entry.
> The other by cleaning up the hash table when the entries array is
> cleared, so that we don't have "pre-inserted" entries.
> 
> Thanks to Eric Dumazet for reminding me of the memory barriers.

Fix the comment below and you can add:
Signed-off-by: Ian Kumlien <pomac@...or.com>

It's currently been running for the longest period ever, ie, 11 minutes
=)

I'm gonna leave it running during the night and send a status update
when the evil daystar reaches it's peak CET. (i haven't been able to
stop since linus mentioned it... damn it... =))

> Signed-off-by: Björn Steinbrink <B.Steinbrink@....de>
> ---

> @@ -360,6 +364,7 @@ static ssize_t tstats_write(struct file *file, const char __user *buf,
>  		if (!active) {
>  			reset_entries();
>  			time_start = ktime_get();
> +			smb_mb();

smb? you mean smp =)

>  			active = 1;
>  		}
>  		break;

-- 
Ian Kumlien <pomac () vapor ! com> -- http://pomac.netswarm.net
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