Linux 内核的 LOCK_PREFIX 的含义

}
。。。
}
上面的代码说，如果是单处理器(UP)，就调这个：
static void alternatives_smp_unlock(u8 **start, u8 **end, u8 *text, u8
*text_end)
{
u8 **ptr;
char insn[1];
终于弄明白了 Linux 内核的 LOCK_PREFIX 的含义
x86 架构的内核实现原子整数的时候，用到了 LOCK_PREFIX 这个宏
static __inline__ void atomic_add(int i, atomic_t *v)
{
__asm__ _
顺手搜了一下：
/search?hl=zh-CN&newwindow=1&q=alternative+kernel+smp&meta=&
aq=f&oq=
又看到这段注释……，本来仔细看一下就明白了，折腾啊。
/*
* Alternative inline assembly for SMP.
apply_alternatives(aseg, aseg + alt->sh_size);
}
if (locks && text) {
void *lseg = (void *)locks->sh_addr;
void alternatives_smp_module_add(struct module *mod, char *name,
void *locks, void *locks_end,
void *text, void *text_end)
LOCK_PREFIX "addl %1,%0"
:"+m" (v->counter)
:"ir" (i));
}
在 CONFIG_SMP 的时候：
#define LOCK_PREFIX \
".section .smp_locks,\"a\"\n" \
四个字节的 lock 前缀的地址，链接的时候，所有的 .smp_locks 段合并起来，形成一个
所有 lock 指令地址的数组，这样统计 .smp_locks 段就能知道代码里有多少个加锁的
指令被生成，猜测是为了调试目的。
----
搜索完成，果然找到了引用处：
linux-2.6.23.12\arch\i386\kernel\module.c
* alternatives and the X86_FEATURE_UP flag to indicate the case of a
* UP system running a SMP kernel. The existing apply_alternatives()
* works fine for patching a SMP kernel for UP.
continue;
if (*ptr > text_end)
continue;
text_poke(*ptr, insn, 1);
};
* useful in virtualized environments.
*
* The very common lock prefix is handled as special case in a
* separate table which is a pure address list without replacement ptr
tseg, tseg + text->sh_size);
}
if (para) {
void *pseg = (void *)para->sh_addr;
if (!strcmp(".text", secstrings + s->sh_name))
text = s;
if (!strcmp(".altinstructions", secstrings + s->sh_name))
当一个内核模块被加载后调用这个
int module_finalize(const Elf_Ehdr *hdr,
const Elf_Shdr *sechdrs,
struct module *me)
{
const Elf_Shdr *s, *text = NULL, *alt = NULL, *locks = NULL,
* and size information. That keeps the table sizes small.
*/
apply_paravirt(pseg, pseg + para->sh_size);
}
return module_bug_finalize(hdr, sechdrs, me);
}
上面的代码说，如果模块有 .text 和 .smp_locks 段，就调这个来处理，做什么呢？
void *tseg = (void *)text->sh_addr;
alternatives_smp_module_add(me, me->name,
lseg, lseg + locks->sh_size,
{
struct smp_alt_module *smp;
unsigned long flags;
if (noreplace_smp)
return;
if (smp_alt_once) {
if (boot_cpu_has(X86_FEATURE_UP))
"661:\n\tlock; "
展开后变成这样：
.section .smp_locks,"a"
.align 4
.long 661f
.previous
661:
lock;
本来觉得直接加上 lock 前缀即可，前面一堆干吗用的一直不明白，终于决定要搞懂，
*para = NULL;
char *secstrings = (void *)hdr + sechdrs[hdr->e_shstrndx].sh_offset;
for (s = sechdrs; s < sechdrs + hdr->e_shnum; s++) {
alternatives_smp_unlock(locks, locks_end,
text, text_end);
return;
if (noreplace_smp)
return;
add_nops(insn, 1);
for (ptr = start; ptr < end; ptr++) {
if (*ptr < text)
para = s;
}
if (alt) {
/* patch .altinstructions */
void *aseg = (void *)alt->sh_addr;
*
* The LOCK_PREFIX macro defined here replaces the LOCK and
* LOCK_PREFIX macros used everywhere in the source tree.
*
* SMP alternatives use the same data structures as the other
}
看到这里就能明白，这是内核配置了 smp，但是实际运行到单处理器上时，通过运行期间
打补丁，根据 .smp_locks 里的记录，把 lock 指令前缀替换成 nop 以消除指令加锁的
开销，这个优化真是极致了……，可能考虑很多用户直接使用的是配置支持 SMP 编译好
的内核而特地对 x86/x64 做的这个优化。
*
* The SMP alternative tables can be kept after boot and contain both
* UP and SMP versions of the instructions to allow switching back to
* SMP at runtime, when hotplugging in a new CPU, which is especially
" .align 4\n" \
" .long 661f\n" /* address */ \
".previous\n" \
alt = s;
if (!strcmp(".smp_locks", secstrings + s->sh_name))
locks= s;
if (!strcmp(".parainstructions", secstrings + s->sh_name))
翻开 as 手册，查了个明白，现逐条解释如下：
.section .smp_locks,"a"
下面的代码生成到 .smp_locks 段里，属性为"a", allocatable，参考 as 7.76 .section