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

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Message-ID: <4D801F3A.4000303@cn.fujitsu.com>
Date:	Wed, 16 Mar 2011 10:23:54 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Arnd Bergmann <arnd@...db.de>
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 03/15/2011 06:15 PM, Arnd Bergmann wrote:
> 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.

The first one is not a problem nor assumption, if an rcu_head offset is larger
than 4096, the BUILD_BUG_ON() will be triggered, and the user can use the original
call_rcu() instead.

b) is not a problem, the TEXT section is no at the first 4 KB of virtual address space.

> 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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