linux-kernel - Re: Major regression on hackbench with SLUB (more numbers)

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Date:	Wed, 26 Dec 2007 21:51:42 -0800 (PST)
From:	Christoph Lameter <clameter@....com>
To:	Al Viro <viro@...IV.linux.org.uk>
cc:	Theodore Tso <tytso@....edu>, Andi Kleen <andi@...stfloor.org>,
	Willy Tarreau <w@....eu>, Steven Rostedt <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Hellwig <hch@...radead.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: Major regression on hackbench with SLUB (more numbers)

On Wed, 26 Dec 2007, Al Viro wrote:

> Erm...  Let me spell it out: current lifetime rules are completely broken.
> As it is, create/destroy/create cache sequence will do kobject_put() on
> kfree'd object.  Even without people playing with holding sysfs files
> open or doing IO on those.

Urgh. Help! Isnt there some sanity with sysfs? 

> a) you have kobject embedded into struct with the lifetime rules of its
> own.  When its refcount hits zero you kfree() the sucker, even if you
> still have references to embedded kobject.

So alloate it separately?

> b) your symlinks stick around.  Even when cache is long gone you still
> have a sysfs symlink with its embedded kobject as a target.  They are
> eventually removed when cache with the same name gets created.  _Then_
> you get the target kobject dropped - when the memory it used to be in
> had been freed for hell knows how long and reused by something that would
> not appreciate slub.c code suddenly deciding to decrement some word in
> that memory.

Hmmmm.. That would also be addressed by a having a separately allocated 
kobject?
 
> c) you leak references to these kobject; kobject_del() only removes it
> from the tree undoing the effect of kobject_add() and you still need
> kobject_put() to deal with the last reference.

Patches would be appreciated.... Had a hard time to get the sysfs support 
to work right.
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