linux-kernel - Re: 2.6.39-rc6-mmotm0506 and -next - __kfree

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Date:	Mon, 09 May 2011 13:02:16 -0400
From:	Valdis.Kletnieks@...edu
To:	unlisted-recipients:; (no To-header on input)
Cc:	Randy Dunlap <randy.dunlap@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	linux-kernel@...r.kernel.org
Subject: Re: 2.6.39-rc6-mmotm0506 and -next - __kfree_rcu breaks third-party kernel code

On Mon, 09 May 2011 12:56:32 EDT, Valdis.Kletnieks@...edu said:

> OK, *now* I'm confused.  The magic sauce appears to be "-Os". Adding that makes
> the compile work.  And for no obvious reason - there's something in there that
> causes indigestion, but only if the optimizer isn't invoked.

/lib/modules/2.6.39-rc6-mmotm0506//build/include/linux/rcupdate.h: In function '__kfree_rcu':
/lib/modules/2.6.39-rc6-mmotm0506//build/include/linux/rcupdate.h:822:2: error: size of unnamed array is negative

Apparently, it's in fact bad code - but the entire __kfree_rcu() function is
being reaped by the optimizer as dead code before the code generator
sees the size issue with the array.  Without optimization, the code hangs
around long enough for the problem to crop up.


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