redonkable/remarkable-linux
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Peter Zijlstra 3c8ed88974 kref: Remove the memory barriers 
Commit 1b0b3b9980 ("kref: fix CPU ordering with respect to krefs")
wrongly adds memory barriers to kref.

It states:

  some atomic operations are only atomic, not ordered. Thus a CPU is allowed
  to reorder memory references to an object to before the reference is
  obtained. This fixes it.

While true, it fails to show why this is a problem. I say it is not a
problem because if there is a race with kref_put() such that we could
end up referencing a free'd object without this memory barrier, we
would still have that race with the memory barrier.

The kref_put() in question could complete (and free the object) before
the atomic_inc() and we'd still be up shit creek.

The kref_init() case is even worse, if your object is published at this
time you're so wrong the memory barrier won't make a difference what
so ever. If its not published, the act of publishing should include
the needed barriers/locks to make sure all writes prior to the act of
publishing are complete such that others will only observe a complete
object.

Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
2011-12-13 09:11:19 -08:00
..
acpi Merge branch 'release' of git://git.kernel.org/pub/scm/linux/kernel/git/lenb/linux 2011-11-07 10:13:52 -08:00
asm-generic Merge Qualcom Hexagon architecture 2011-11-01 07:48:13 -07:00
crypto crypto: Add a report function pointer to crypto_type 2011-10-21 14:24:03 +02:00
drm Merge branch 'drm-fixes' of git://people.freedesktop.org/~airlied/linux 2011-11-07 10:01:56 -08:00
keys
linux kref: Remove the memory barriers 2011-12-13 09:11:19 -08:00
math-emu
media Merge branch 'modsplit-Oct31_2011' of git://git.kernel.org/pub/scm/linux/kernel/git/paulg/linux 2011-11-06 19:44:47 -08:00
misc [media] altera-stapl: it is time to move out from staging 2011-09-23 15:00:57 -03:00
mtd mtd: kill old field for `struct mtd_info_user' 2011-09-11 15:57:44 +03:00
net Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net 2011-11-07 10:55:33 -08:00
pcmcia pcmcia: add PCMCIA_DEVICE_MANF_CARD_PROD_ID3 2011-08-08 16:04:11 -04:00
rdma Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/roland/infiniband 2011-11-01 10:51:38 -07:00
rxrpc atomic: use <linux/atomic.h> 2011-07-26 16:49:47 -07:00
scsi Merge git://git.kernel.org/pub/scm/linux/kernel/git/jejb/scsi-misc-2.6 2011-10-28 16:44:18 -07:00
sound Merge branch 'modsplit-Oct31_2011' of git://git.kernel.org/pub/scm/linux/kernel/git/paulg/linux 2011-11-06 19:44:47 -08:00
target Merge branch 'for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/nab/target-pending 2011-11-06 19:00:42 -08:00
trace Merge branch 'modsplit-Oct31_2011' of git://git.kernel.org/pub/scm/linux/kernel/git/paulg/linux 2011-11-06 19:44:47 -08:00
video Merge branch 'for-florian' of git://gitorious.org/linux-omap-dss2/linux into fbdev-next 2011-10-15 00:19:52 +00:00
xen Merge branch 'upstream/xen-settime' of git://git.kernel.org/pub/scm/linux/kernel/git/jeremy/xen 2011-11-06 20:15:05 -08:00
Kbuild