netdev - Re: [PATCH v3 2/2] decrement static keys on real destroy time

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Date:	Thu, 26 Apr 2012 15:13:24 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Glauber Costa <glommer@...allels.com>
Cc:	cgroups@...r.kernel.org, netdev@...r.kernel.org,
	Li Zefan <lizefan@...wei.com>, kamezawa.hiroyu@...fujitsu.com,
	linux-mm@...ck.org, devel@...nvz.org
Subject: Re: [PATCH v3 2/2] decrement static keys on real destroy time

Hello, Glauber.

On Thu, Apr 26, 2012 at 06:58:37PM -0300, Glauber Costa wrote:
> At first I though that we could get rid of all this complication by
> calling stop machine from the static_branch API. This would all
> magically go away. I actually even tried it.
> 
> However, reading the code for other architectures (other than x86),
> I found that they usually rely on the fixed instruction size to just
> patch an instruction atomically and go home happy.
> 
> Using stop machine and the like would slow them down considerably.
> Not only slow down the static branch update (which is acceptable),
> but everybody else (which is horrible). It seemed to defeat the
> purpose of static branches a bit.
> 
> The other users of static branches seems to be fine coping with the
> fact that in cases with multiple-sites, they will spread in time.

No, what I mean is that why can't you do about the same mutexed
activated inside static_key API function instead of requiring every
user to worry about the function returning asynchronously.
ie. synchronize inside static_key API instead of in the callers.

Thanks.

-- 
tejun
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