Re: [PATCH] fix vmstat per cpu usage

Jan Blunck
2006-08-07T12:42:32Z

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On Wed, Aug 02, Andrew Morton wrote:

> > +#define per_cpu(var, cpu) (*({ \
> > + int user_error_##var __attribute__ ((unused)); \
> > + RELOC_HIDE(&per_cpu__##var, __per_cpu_offset[cpu]); }))
>
> What's it do? Forces a syntax error if `var' isn't a simple identifier?
>
> Seems sane, although I'd check that the compiler doesn't accidentally
> waste a stack slot for that local. Perhaps it's be safer to make
> it a non-existing function:
>
> extern int user_error#var(void);
>

Yes, that is even better. See attached patch with the modifications for my
major archs (s390, x86-64, generic). I did a run of 'make allmodconfig' with
the attached patch and it produced some errors. I'll send the patches in a
seperate mail.

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From: Jan Blunck <jblunck@suse.de>
Subject: trigger a syntax error if percpu macros are incorrectly used

get_cpu_var()/per_cpu()/__get_cpu_var() arguments must be simple identifiers.
Otherwise the arch dependant implementations might break. This patch enforces
the correct usage of the macros by producing a syntax error if the variable is
not a simple identifier.

Signed-off-by: Jan Blunck <jblunck@suse.de>
---
include/asm-generic/percpu.h | 4 +++-
include/asm-s390/percpu.h | 20 +++++++++++---------
include/asm-x86_64/percpu.h | 12 +++++++++---
include/linux/percpu.h | 10 ++++++++--
4 files changed, 31 insertions(+), 15 deletions(-)

Index: linux-2.6/include/asm-generic/percpu.h
===================================================================
--- linux-2.6.orig/include/asm-generic/percpu.h
+++ linux-2.6/include/asm-generic/percpu.h
@@ -14,7 +14,9 @@ extern unsigned long __per_cpu_offset[NR
__attribute__((__section__(".data.percpu"))) __typeof__(type) per_cpu__##name

/* var is in discarded region: offset to particular copy we want */
-#define per_cpu(var, cpu) (*RELOC_HIDE(&per_cpu__##var, __per_cpu_offset[cpu]))
+#define per_cpu(var, cpu) (*({ \
+ extern int simple_indentifier_##var(void); \
+ RELOC_HIDE(&per_cpu__##var, __per_cpu_offset[cpu]); }))
#define __get_cpu_var(var) per_cpu(var, smp_processor_id())
#define __raw_get_cpu_var(var) per_cpu(var, raw_smp_processor_id())

Index: linux-2.6/include/asm-s390/percpu.h
===================================================================
--- linux-2.6.orig/include/asm-s390/percpu.h
+++ linux-2.6/include/asm-s390/percpu.h
@@ -15,18 +15,20 @@
*/
#if defined(__s390x__) && defined(MODULE)

-#define __reloc_hide(var,offset) \
- (*({ unsigned long *__ptr; \
- asm ( "larl %0,per_cpu__"#var"@GOTENT" \
- : "=a" (__ptr) : "X" (per_cpu__##var) ); \
- (typeof(&per_cpu__##var))((*__ptr) + (offset)); }))
+#define __reloc_hide(var,offset) (*({ \
+ extern int simple_indentifier_##var(void); \
+ unsigned long *__ptr; \
+ asm ( "larl %0,per_cpu__"#var"@GOTENT" \
+ : "=a" (__ptr) : "X" (per_cpu__##var) ); \
+ (typeof(&per_cpu__##var))((*__ptr) + (offset)); }))

#else

-#define __reloc_hide(var, offset) \
- (*({ unsigned long __ptr; \
- asm ( "" : "=a" (__ptr) : "0" (&per_cpu__##var) ); \
- (typeof(&per_cpu__##var)) (__ptr + (offset)); }))
+#define __reloc_hide(var, offset) (*({ \
+ extern int simple_indentifier_##var(void); \
+ unsigned long __ptr; \
+ asm ( "" : "=a" (__ptr) : "0" (&per_cpu__##var) ); \
+ (typeof(&per_cpu__##var)) (__ptr + (offset)); }))

#endif

Index: linux-2.6/include/asm-x86_64/percpu.h
===================================================================
--- linux-2.6.orig/include/asm-x86_64/percpu.h
+++ linux-2.6/include/asm-x86_64/percpu.h
@@ -21,9 +21,15 @@
__attribute__((__section__(".data.percpu"))) __typeof__(type) per_cpu__##name

/* var is in discarded region: offset to particular copy we want */
-#define per_cpu(var, cpu) (*RELOC_HIDE(&per_cpu__##var, __per_cpu_offset(cpu)))
-#define __get_cpu_var(var) (*RELOC_HIDE(&per_cpu__##var, __my_cpu_offset()))
-#define __raw_get_cpu_var(var) (*RELOC_HIDE(&per_cpu__##var, __my_cpu_offset()))
+#define per_cpu(var, cpu) (*({ \
+ extern int simple_indentifier_##var(void); \
+ RELOC_HIDE(&per_cpu__##var, __per_cpu_offset(cpu)); }))
+#define __get_cpu_var(var) (*({ \
+ extern int simple_indentifier_##var(void); \
+ RELOC_HIDE(&per_cpu__##var, __my_cpu_offset()); }))
+#define __raw_get_cpu_var(var) (*({ \
+ extern int simple_indentifier_##var(void); \
+ RELOC_HIDE(&per_cpu__##var, __my_cpu_offset()); }))

/* A macro to avoid #include hell... */
#define percpu_modcopy(pcpudst, src, size) \
Index: linux-2.6/include/linux/percpu.h
===================================================================
--- linux-2.6.orig/include/linux/percpu.h
+++ linux-2.6/include/linux/percpu.h
@@ -11,8 +11,14 @@
#define PERCPU_ENOUGH_ROOM 32768
#endif

-/* Must be an lvalue. */
-#define get_cpu_var(var) (*({ preempt_disable(); &__get_cpu_var(var); }))
+/*
+ * Must be an lvalue. Since @var must be a simple identifier,
+ * we force a syntax error here if it isn't.
+ */
+#define get_cpu_var(var) (*({ \
+ extern int simple_indentifier_##var(void); \
+ preempt_disable(); \
+ &__get_cpu_var(var); }))
#define put_cpu_var(var) preempt_enable()

#ifdef CONFIG_SMP