netdev - RE: [PATCH net-next] r8169: Add 64bit statistics

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Date:	Thu, 17 Nov 2011 15:27:58 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	Junchang Wang <junchangwang@...il.com>,
	Stephen Hemminger <stephen.hemminger@...tta.com>,
	netdev@...r.kernel.org, romieu@...zoreil.com,
	nic swsd <nic_swsd@...ltek.com>
Subject: RE: [PATCH net-next] r8169: Add 64bit statistics

Le jeudi 17 novembre 2011 à 14:01 +0000, David Laight a écrit :

> Ah yes ...
> 
> Both u64_stats_update_begin & _end increment a numeric field
> with an appropriate memory barrier. So the 'update' path
> has two extra read-modify-write sequences (possibly the
> 2nd read can be optimised out), and two smp_wmb() that may
> introduce bus delays.
> 
> Would be fine if it were guaranteed to work!
> Consider the following sequence of events:
>        u64_stats_update_begin()
>        calculate 'count+1'
>                                 read_seqcount_begin()
>                                 read count_hi
>        write count_lo
>                                 read count_lo
>        write count_hi
>                                 read_seqcount_retry()
>        ... update other counters ...
>        u64_stats_update_end()
> The reader gets an invalid value since it reads the same
> 'sequence' both times.
> 
> Could be fixed by using '|= 1' in update_begin and
> looping on odd values in read_seqcount_begin().
> 

I am a bit tired of this discussion.

You obviously dont understand how the thing is working.

Spend some time on it before claiming there are bugs.



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