netdev - Re: [PATCH RFC 1/2] flow: virtualize get and entry deletion methods

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Message-ID: <20100329111025.GA23927@gondor.apana.org.au>
Date:	Mon, 29 Mar 2010 19:10:25 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Timo Teräs <timo.teras@....fi>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH RFC 1/2] flow: virtualize get and entry deletion methods

On Mon, Mar 29, 2010 at 01:36:40PM +0300, Timo Teräs wrote:
>
>>> We can still drop the locking, as ->dead can be made atomic_t.
>>
>> No it doesn't need to be atomic, reading an int is always atomic.
>
> The only reason why it needs to be atomic is because of
> xfrm_policy_kill() which writes '1' and checks if it was zero
> previously. Since the idea is to get rid of the policy lock, we
> can turn ->dead flag to atomic_t and use atomic_xchg for that.
> Otherwise it would be ok to have it as just regular int.

I don't see the point.  As long as the data paths do not take
the lock changing this doesn't buy us much.  You're still making
that cacheline exclusive.

Thanks,
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