ARM CPU Hotplug breakage on mach-shmobile and KZM9D

[Simon Horman]

[ Cc Paul Gortmaker ]

On Thu, Jul 25, 2013 at 10:06:07AM +0900, Simon Horman wrote:
> On Thu, Jul 25, 2013 at 12:15:49AM +0900, Magnus Damm wrote:
> > Hi everyone,
> > 
> > Bad news: CPU Hotplug on KZM9D seems broken on recent kernels.
> > 
> > On renesas-next-20130710 or earlier I can perform the following:
> > 
> > # echo 0 > /sys/devices/system/cpu/cpu1/online
> > CPU1: shutdown
> > # echo 1 > /sys/devices/system/cpu/cpu1/online
> > CPU1: Booted secondary processor
> > #
> > 
> > While on more recent kernels the hotplug boot fails. This may be
> > related to the move to v3.11-rc. The tag renesas-next-20130710
> > surprisingly seems to be the final kernel based on v3.10-rc before
> > jumping to v3.11-rc.
> > 
> > I wonder, do other ARM platforms have working CPU Hotplug in v3.11-rc?
> 
> I have a snowball that I could test if you think it would be useful.

Unfortunately getting snowball to work turned out to be rather painful.
So instead I bisected the problem on the KZM9G (N.b: the D in the subject
seems to be incorrect :^)

The result is that the following patch, added in v3.11-rc1,
appears to be the culprit.

commit 22f0a27367742f65130c0fb25ef00f7297e032c1
Author: Paul Gortmaker <paul.gortmaker at windriver.com>
Date:   Mon Jun 17 18:34:14 2013 -0400

    init.h: remove __cpuinit sections from the kernel
    
    The __cpuinit type of throwaway sections might have made sense
    some time ago when RAM was more constrained, but now the savings
    do not offset the cost and complications.  For example, the fix in
    commit 5e427ec2d0 ("x86: Fix bit corruption at CPU resume time")
    is a good example of the nasty type of bugs that can be created
    with improper use of the various __init prefixes.
    
    After a discussion on LKML[1] it was decided that cpuinit should go
    the way of devinit and be phased out.  Once all the users are gone,
    we can then finally remove the macros themselves from linux/init.h.
    
    As an interim step, we can dummy out the macros to be no-ops, and
    this will allow us to avoid a giant tree-wide patch, and instead
    we can feed in smaller chunks mainly via the arch/ trees.  This
    is in keeping with commit 78d86c213f28193082b5d8a1a424044b7ba406f1
    ("init.h: Remove __dev* sections from the kernel")
    
    We don't strictly need to dummy out the macros to do this, but if
    we don't then some harmless section mismatch warnings may temporarily
    result.  For example, notify_cpu_starting() and cpu_up() are arch
    independent (kernel/cpu.c) and are flagged as __cpuinit.  And hence
    the calling functions in the arch specific code are also expected
    to be __cpuinit -- if not, then we get the section mismatch warning.
    
    Two of the three __CPUINIT variants are not used whatsoever, and
    so they are simply removed directly at this point in time.
    
    [1] https://lkml.org/lkml/2013/5/20/589
    
    Signed-off-by: Paul Gortmaker <paul.gortmaker at windriver.com>

diff --git a/include/linux/init.h b/include/linux/init.h
index 8618147..e73f2b70 100644
--- a/include/linux/init.h
+++ b/include/linux/init.h
@@ -93,13 +93,13 @@
 
 #define __exit          __section(.exit.text) __exitused __cold notrace
 
-/* Used for HOTPLUG_CPU */
-#define __cpuinit        __section(.cpuinit.text) __cold notrace
-#define __cpuinitdata    __section(.cpuinit.data)
-#define __cpuinitconst   __constsection(.cpuinit.rodata)
-#define __cpuexit        __section(.cpuexit.text) __exitused __cold notrace
-#define __cpuexitdata    __section(.cpuexit.data)
-#define __cpuexitconst   __constsection(.cpuexit.rodata)
+/* temporary, until all users are removed */
+#define __cpuinit
+#define __cpuinitdata
+#define __cpuinitconst
+#define __cpuexit
+#define __cpuexitdata
+#define __cpuexitconst
 
 /* Used for MEMORY_HOTPLUG */
 #define __meminit        __section(.meminit.text) __cold notrace
@@ -118,9 +118,8 @@
 #define __INITRODATA	.section	".init.rodata","a",%progbits
 #define __FINITDATA	.previous
 
-#define __CPUINIT        .section	".cpuinit.text", "ax"
-#define __CPUINITDATA    .section	".cpuinit.data", "aw"
-#define __CPUINITRODATA  .section	".cpuinit.rodata", "a"
+/* temporary, until all users are removed */
+#define __CPUINIT
 
 #define __MEMINIT        .section	".meminit.text", "ax"
 #define __MEMINITDATA    .section	".meminit.data", "aw"