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

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Date:	Mon, 29 Mar 2010 14:23:02 +0300
From:	Timo Teräs <timo.teras@....fi>
To:	Herbert Xu <herbert@...dor.apana.org.au>
CC:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH RFC 1/2] flow: virtualize get and entry deletion methods

Herbert Xu wrote:
> 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.

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.

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.
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