linux-kernel - Re: [PATCH] drivers/net: Use RCU_INIT

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Message-ID: <1395665638.12610.9.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Mon, 24 Mar 2014 05:53:58 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Monam Agarwal <monamagarwal123@...il.com>, davem@...emloft.net,
	jasowang@...hat.com, xemul@...allels.com, wuzhy@...ux.vnet.ibm.com,
	therbert@...gle.com, yamato@...hat.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drivers/net: Use RCU_INIT_POINTER(x, NULL) in tun.c

On Mon, 2014-03-24 at 08:22 +0200, Michael S. Tsirkin wrote:

> What happens if someone then changes that NULL to something else?
> Things will start to break in subtle way, won't they?

Sure. As anything else can break when/if using wrong API or in any
change. Particularly in RCU protected code. Even a kzalloc() can be
buggy.

Note that smp_wmb() is hardly expensive in all these paths,
so thats why I do not care anymore asking guys using RCU_INIT_POINTER(x,
NULL) instead of rcu_assign_pointer() as I used to.

When doing tree wide change (like in commit 2cfa5a0471fef43), I usually
not do a dumb one, I take the time to read the code and check it for
defects.

I do not remember what kind of compiler warnings we had 3 years ago,
But I do not want to spend more cycles on this subject...

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