[PATCH] __GENERIC_PER_CPU changes

Now CONFIG_DEBUG_INITDATA is in, initial percpu data
[__per_cpu_start,__per_cpu_end] can be declared as a redzone, and invalid
accesses after boot can be detected, at least for i386.

We can let non possible cpus percpu data point to this 'redzone' instead of
NULL .

NULL was not a good choice because part of [0..32768] memory may be
readable and invalid accesses may happen unnoticed.

If CONFIG_DEBUG_INITDATA is not defined, each non possible cpu points to
the initial percpu data (__per_cpu_offset[cpu] == 0), thus invalid accesses
wont be detected/crash.

This patch also moves __per_cpu_offset[] to read_mostly area to avoid false
sharing.
Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

init/main.c

@@ -325,7 +325,7 @@ static inline void smp_prepare_cpus(unsigned int maxcpus) { }
 #else
 
 #ifdef __GENERIC_PER_CPU
-unsigned long __per_cpu_offset[NR_CPUS];
+unsigned long __per_cpu_offset[NR_CPUS] __read_mostly;
 
 EXPORT_SYMBOL(__per_cpu_offset);
 
@@ -343,11 +343,7 @@ static void __init setup_per_cpu_areas(void)
 #endif
 	ptr = alloc_bootmem(size * nr_possible_cpus);
 
-	for (i = 0; i < NR_CPUS; i++) {
-		if (!cpu_possible(i)) {
-			__per_cpu_offset[i] = (char*)0 - __per_cpu_start;
-			continue;
-		}
+	for_each_cpu(i) {
 		__per_cpu_offset[i] = ptr - __per_cpu_start;
 		memcpy(ptr, __per_cpu_start, __per_cpu_end - __per_cpu_start);
 		ptr += size;