linux-kernel - Re: [PATCH V4 1/1] rcu: introduce kfree

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Message-Id: <201103151115.54426.arnd@arndb.de>
Date:	Tue, 15 Mar 2011 11:15:54 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Manfred Spraul <manfred@...orfullife.com>
Subject: Re: [PATCH V4 1/1] rcu: introduce kfree_rcu()

On Tuesday 15 March 2011 10:46:20 Lai Jiangshan wrote:
> +static __always_inline bool __is_kfree_rcu_offset(unsigned long offset)
> +{
> +       return offset < 4096;
> +}

So this relies on the assumptions that 

a) the rcu_head is within the first 4 KB of the data structure to be freed
b) no callback ever gets called in the first 4 KB of virtual address space

It's probably a reasonable assumption, but I think it should be documented
more explicitly, especially the first one. It's entirely possible that
an RCU managed data structure is larger than 4 KB.
Another alternative might be to encode the difference between a
function pointer and an offset in one of the lower bits of the address.

	Arnd
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