netdev - Re: [PATCH] ppp: add 64 bit stats

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Message-ID: <CABF+-6VAWVM=Cpyjk4iTbG-bBfHUs_CfRhe7rXmxxiF_ZX7_OA@mail.gmail.com>
Date:	Wed, 25 Jul 2012 10:43:36 -0400
From:	Kevin Groeneveld <kgroeneveld@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH] ppp: add 64 bit stats

On Wed, Jul 25, 2012 at 12:04 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> It there are spinlocks already, why even adding u64_stats_sync ?

That is a good question I have already wondered about myself.

>include/linux/u64_stats_sync.h
>
>* 3) Write side must ensure mutual exclusion or one seqcount update could
> *    be lost, thus blocking readers forever.
> *    If this synchronization point is not a mutex, but a spinlock or
> *    spinlock_bh() or disable_bh() :

It seems the u64_stats_sync requires some form of mutual exclusion.
So why bother ever using it at all?  Maybe there are cases where the
required mutual exclusion can be cheaper than a spinlock?  Maybe it is
just to avoid the spinlocks on the read side of things?

I hope you don't mind all my questions...
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