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

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Date:	Mon, 29 Mar 2010 19:32:25 +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 02:23:02PM +0300, Timo Teräs wrote:
>
>> 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.
>
> To my understanding declaring an atomic_t, or reading it with
> atomic_read does not make cache line exclusive. Only the atomic_*
> writing to it take the cache line. And since this is done exactly
> once for policy (or it's a bug/warn thingy) it does not impose
> significant performance issue.

I was talking about the lock vs. atomic_xchg in xfrm_policy_kill.
There is practically no difference for that case.

Yes, on the read side the lock is a completely different beast
compared to atomic_read, but I don't see how you can safely
replace

	lock
	if (!dead)
		take ref
	unlock

without making other changes.

> But looking at the code more. The check should not be needed.
> xfrm_policy_kill() is only called if the entry is removed from
> the hash list, which can happen only once.
>
> Do you think we can just change it to unconditionally writing
> to "policy->walk.dead = 1;" and be done with that?
>
> Alternatively, we can move the ->dead check to be done while
> holding the hash lock to guarantee no one else is writing
> simultaneously.

See above.

Cheers,
-- 
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Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
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