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

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Message-ID: <20110316040203.GB2273@linux.vnet.ibm.com>
Date:	Tue, 15 Mar 2011 21:02:03 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Lai Jiangshan <laijs@...fujitsu.com>, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Manfred Spraul <manfred@...orfullife.com>
Subject: Re: [PATCH V4 1/1] rcu: introduce kfree_rcu()

On Tue, Mar 15, 2011 at 02:07:24PM +0100, Arnd Bergmann wrote:
> On Tuesday 15 March 2011, Paul E. McKenney wrote:
> > And it makes use of statically allocated structures a bit clunky.
> 
> How do statically allocated structures relate to this? I would
> expect that you never call kfree_rcu on them, so it shouldn't
> matter.
> 
> > Yet another approach is to use the low-order bit of the rcu_head pointer,
> > given that the rcu_head structure does have to be aligned.  If this bit
> > is set, then the function pointer could be interpreted as an offset.
> > This approach might also allow a slab_free_rcu() to be constructed, given
> > that the full 32 bits of the function pointer would be available.
> > For example, if the upper 16 bits are zero, the low-order 16 bits are
> > the offset.  If the upper 16 bits are 0x1, then the low-order 16 bits
> > might be an index that selects the desired slab cache.
> 
> This solution sounds like a clear improvement over the patch that Lai
> Jiangshan posted, without any downsides.

Except that I was forgetting that we don't really have any way to stop
people from handing us misaligned rcu_head structures -- that topic came
up last time as well.  Or were the people mentioning that possibility
being overly paranoid?

							Thanx, Paul
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