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SubjectRe: [PATCH] 2.5.1-pre5: per-cpu areas
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In message <15504.7958.677592.908691@napali.hpl.hp.com> you write:
> OK, I see this. Unfortunately, HIDE_RELOC() causes me problems
> because it prevents me from taking the address of a per-CPU variable.
> This is useful when you have a per-CPU structure (e.g., cpu_info).
> Perhaps there should/could be a version of HIDE_RELOC() that doesn't
> dereference the resulting address?

Yep, valid point. Patch below: please play.

> I am also a bit concerned however about aliasing that the compiler
> might not detect. For example, with this code:
>
> this_cpu(foo) = 13;
> per_cpu(foo, 0) = 15;
> printf("foo=%d\n", this_cpu(foo);
>
> might print the wrong value if gcc thinks that the first and second
> assignment never alias each other. Does HIDE_RELOC() take care of
> this also?

I'd be pretty sure the compiler can't assume that. Richard would
know...

> On a side-note, would you mind moving __per_cpu_data from smp.h into
> compiler.h? I'd like to use it in processor.h and from that file, I
> can't include smp.h due to a recursive dependency.

I had to fight a similar #include battle with sched.h recently.

I came out with the conclusion that it's better to create a new
include than shuffle inappropriate stuff into other includes to solve
these kind of issues.

Cheers!
Rusty.
--
Anyone who quotes me in their sig is an idiot. -- Rusty Russell.

diff -urN -I \$.*\$ --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal linux-2.5.7-pre1/include/linux/compiler.h working-2.5.7-pre1-nfarp/include/linux/compiler.h
--- linux-2.5.7-pre1/include/linux/compiler.h Fri Mar 8 14:49:29 2002
+++ working-2.5.7-pre1-nfarp/include/linux/compiler.h Thu Mar 14 15:32:38 2002
@@ -13,10 +13,4 @@
#define likely(x) __builtin_expect((x),1)
#define unlikely(x) __builtin_expect((x),0)

-/* This macro obfuscates arithmetic on a variable address so that gcc
- shouldn't recognize the original var, and make assumptions about it */
-#define RELOC_HIDE(var, off) \
- ({ __typeof__(&(var)) __ptr; \
- __asm__ ("" : "=g"(__ptr) : "0"((void *)&(var) + (off))); \
- *__ptr; })
#endif /* __LINUX_COMPILER_H */
diff -urN -I \$.*\$ --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal linux-2.5.7-pre1/include/linux/percpu.h working-2.5.7-pre1-nfarp/include/linux/percpu.h
--- linux-2.5.7-pre1/include/linux/percpu.h Thu Jan 1 10:00:00 1970
+++ working-2.5.7-pre1-nfarp/include/linux/percpu.h Thu Mar 14 15:32:44 2002
@@ -0,0 +1,28 @@
+#ifndef __LINUX_PERCPU_H
+#define __LINUX_PERCPU_H
+
+/* This macro obfuscates arithmetic on a variable address so that gcc
+ shouldn't recognize the original var, and make assumptions about it */
+#define RELOC_HIDE(ptr, off) \
+ ({ __typeof__(ptr) __ptr; \
+ __asm__ ("" : "=g"(__ptr) : "0"((void *)(ptr) + (off))); \
+ __ptr; })
+
+#ifdef CONFIG_SMP
+#define __per_cpu_data __attribute__((section(".data.percpu")))
+
+#ifndef __HAVE_ARCH_PER_CPU
+extern unsigned long __per_cpu_offset[NR_CPUS];
+
+/* var is in discarded region: offset to particular copy we want */
+#define per_cpu(var, cpu) (*RELOC_HIDE(&var, per_cpu_offset(cpu)))
+
+#define this_cpu(var) per_cpu(var, smp_processor_id())
+#endif /* !__HAVE_ARCH_PER_CPU */
+#else
+#define __per_cpu_data
+#define per_cpu(var, cpu) var
+#define this_cpu(var) var
+#endif /* CONFIG_SMP */
+
+#endif /* __LINUX_PERCPU_H */
diff -urN -I \$.*\$ --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal linux-2.5.7-pre1/include/linux/smp.h working-2.5.7-pre1-nfarp/include/linux/smp.h
--- linux-2.5.7-pre1/include/linux/smp.h Mon Mar 11 14:33:14 2002
+++ working-2.5.7-pre1-nfarp/include/linux/smp.h Thu Mar 14 15:32:23 2002
@@ -72,16 +72,6 @@
#define MSG_RESCHEDULE 0x0003 /* Reschedule request from master CPU*/
#define MSG_CALL_FUNCTION 0x0004 /* Call function on all other CPUs */

-#define __per_cpu_data __attribute__((section(".data.percpu")))
-
-#ifndef __HAVE_ARCH_PER_CPU
-extern unsigned long __per_cpu_offset[NR_CPUS];
-
-/* var is in discarded region: offset to particular copy we want */
-#define per_cpu(var, cpu) RELOC_HIDE(var, per_cpu_offset(cpu))
-
-#define this_cpu(var) per_cpu(var, smp_processor_id())
-#endif /* !__HAVE_ARCH_PER_CPU */
#else /* !SMP */

/*
@@ -101,9 +91,5 @@
#define cpu_online_map 1
static inline void smp_send_reschedule(int cpu) { }
static inline void smp_send_reschedule_all(void) { }
-#define __per_cpu_data
-#define per_cpu(var, cpu) var
-#define this_cpu(var) var
-
#endif
#endif
-
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