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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
This archive was generated by hypermail 2b29 : Fri Mar 15 2002 - 22:00:16 EST