@CopyLeft by ICANTH,I Can do ANy THing that I CAN THink!~
Author:WenHui,WuHan University,2012-6-15
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普通自旋锁
自旋锁最常见的使用场景是创建一段临界区 :
static DEFINE_SPINLOCK(xxx_lock);
unsigned long flags;
spin_lock_irqsave(&xxx_lock, flags);
... critical section here ..
spin_unlock_irqrestore(&xxx_lock, flags);
自旋锁使用时值得注意的是:对于采用使用自旋锁以保证共享变量的存取安全时,仅当系统中所有涉及到存取该共享变量的程序部分都采用成对的spin_lock、和spin_unlock来进行操作才能保证其安全性。
NOTE! The spin-lock is safe only when you _also_ use the lock itself to do locking across CPU's, which implies that EVERYTHING that touches a shared variable has to agree about the spinlock they want to use.
在Linux2.6.15.5中,自旋体数据结构如下:
当配置CONFIG_SMP时,raw_spinlock_t才是一个含有slock变量的结构,该slock字段标识自旋锁是否空闲状态,用以处理多CPU处理器并发申请锁的情况;当未配置CONFIG_SMP时,对于单CPU而言,不会发生发申请自旋锁,故raw_lock为空结构体。
当配置CONFIG_SMP和CONFIG_PREEMPT时,spinlock_t才会有break_lock字段,break_lock字段用于标记自旋锁竞争状态,当break_lock = 0时表示没有多于两个的执行路径,当break_lock = 1时表示没有其它进程在忙等待该锁。当在SMP多CPU体系架构下有可能出现申请不到自旋锁、空等的情况,但LINUX内核必须保证在spin_lock的原子性,故在配置CONFIG_PREEMPT时必须禁止内核抢占。
字段 | 描述 |
spin_lock_init(lock) | 一个自旋锁时,可使用接口函数将其初始化为锁定状态 |
spin_lock(lock) | 用于锁定自旋锁,如果成功则返回;否则循环等待自旋锁变为空闲 |
spin_unlock(lock) | 释放自旋锁lock,重新设置自旋锁为锁定状态 |
spin_is_locked(lock) | 判断当前自旋锁是否处于锁定状态 |
spin_unlock_wait(lock) | 循环等待、直到自旋锁lock变为可用状态 |
spin_trylock(lock) | 尝试锁定自旋锁lock,如不成功则返回0;否则锁定,并返回1 |
spin_can_lock(lock) | 判断自旋锁lock是否处于空闲状态 |
spin_lock和spin_unlock的关系如下:
可见,在UP体系架构中,由于没有必要有实际的锁以防止多CPU抢占,spin操作仅仅是禁止和开启内核抢占。
LINUX 2.6.35版本,将spin lock实现更改为 ticket lock。spin_lock数据结构除了用于内核调试之外,字段为: 。
ticket spinlock将rlock字段分解为如下两部分:
Next是下一个票号,而Owner是允许使用自旋锁的票号。加锁时CPU取Next,并将rlock.Next + 1。将Next与Owner相比较,若相同,则加锁成功;否则循环等待、直到Next = rlock.Owner为止。解锁则直接将Owner + 1即可。
spin_lock和spin_unlock的调用关系如下:
普通自旋锁源码分析
源程序文件目录关系图
在/include/linux/spinlock.h中通过是否配置CONFIG_SMP项判断导入哪种自旋锁定义及操作:
/*
* include/linux/spinlock.h - generic spinlock/rwlock declarations
* here's the role of the various spinlock/rwlock related include files:
* on SMP builds:
* asm/spinlock_types.h: contains the arch_spinlock_t/arch_rwlock_t and the
* initializers
* linux/spinlock_types.h:
* defines the generic type and initializers
* asm/spinlock.h: contains the arch_spin_*()/etc. lowlevel
* implementations, mostly inline assembly code
* linux/spinlock_api_smp.h:
* contains the prototypes for the _spin_*() APIs.
* linux/spinlock.h: builds the final spin_*() APIs.
* on UP builds:
* linux/spinlock_type_up.h:
* contains the generic, simplified UP spinlock type.
* (which is an empty structure on non-debug builds)
* linux/spinlock_types.h:
* defines the generic type and initializers
* linux/spinlock_up.h:
* contains the arch_spin_*()/etc. version of UP
* builds. (which are NOPs on non-debug, non-preempt
* builds)
* (included on UP-non-debug builds:)
* linux/spinlock_api_up.h:
* builds the _spin_*() APIs.
* linux/spinlock.h: builds the final spin_*() APIs.
*/
/*
* Pull the arch_spin*() functions/declarations (UP-nondebug doesnt need them):
*/
#ifdef CONFIG_SMP
# include
#else
# include <>
#endif
typedef struct {
union {
struct ;
};
} ;
static inline void ( *)
{
(&->);
}
#define () ()
static inline void ( *)
{
(&->);
}
#define () ()
UP体系架构
spin_lock函数在UP体系架构中最终实现方式为:
/include/linux/spinlock_api_up.h
#define () ()
/*
* In the UP-nondebug case there's no real locking going on, so the
* only thing we have to do is to keep the preempt counts and irq
* flags straight, to suppress compiler warnings of unused lock
* variables, and to add the proper checker annotations:
*/
#define () \
do { (); (); (void)(); } while (0)
#define () ()
preempt_disable在未配置CONFIG_PREEMPT时为空函数,否则禁止内核抢占。而__acquire()用于内核编译过程中静态检查。(void)(lock)则是为避免编译器产生lock未被使用的警告。
spin_unlock函数在UP体系架构中最终实现方式为:
#define () \
do { (); (); (void)(); } while (0)
SMP体系架构-Tickect Spin Lock的实现方式
在Linux2.6.24中,自旋锁由一个整数表示,当为1时表示锁是空闲的,spin_lock()每次减少1,故 <=0时则表示有多个锁在忙等待,但这将导致不公平性。自linux2.6.25开始,自旋锁将整数拆为一个16位数,结构如下:
该实现机制称为“Ticket spinlocks”,Next字节表示下一次请求锁给其分配的票号,而Owner表示当前可以取得锁的票号,Next和Owner初始化为0。当lock.Next = lock.Owner时,表示该锁处于空闲状态。spin_lock执行如下过程:
1、my_ticket = slock.next
2、slock.next++
3、wait until my_ticket = slock.owner
spin_unlock执行如下过程:
1、slock.owner++
但该锁将导致一个问题:8个bit将只能最多表示255个CPU来竞争该锁。故系统通过的方式,将实现两个tickect_spin_lock和ticket_spin_unclock的版本:
#if ( < 256)
#define 8
#else
#define 16
SMP体系架构-SPIN LOCK (ticket_shif 8)
#ifdef CONFIG_INLINE_SPIN_LOCK
#define () ()
#endif
/include/linux/spinlock_api_smp.h:
static inline void ( *)
{
();
(&->, 0, 0, );
(, , );
}
在__raw_spin_lock中,首先禁止内核抢占,调用LOCK_CONTENED宏
#define (, try, ) \
do { \
if (!try()) { \
(&()->, ); \
(); \
} \
(&()->, ); \
} while (0)
其中即在_raw_spin_lock中,即为首先调用do_raw_spin_trylock尝试加锁,若失败则继续调用do_raw_spin_lock进行加锁。而do_raw_spin_xxx具体实现与平台有关。
/include/linux/spinlock.h
static inline void ( *) ()
{
();
(&->);
}
static inline int ( *)
{
return (&()->);
}
在X86平台下,和实现为两个函数:
/arch/x86/include/asm/spinlock.h
static void ( *)
{
();
}
static int ( *)
{
return ();
}
#if ( < 256)
#define 8
static void ( *)
{
short = 0x0100;
asm volatile (
"xaddw %w0, %1\n"
"1:\t"
"cmpb %h0, %b0\n\t"
"je 2f\n\t"
"rep ; nop\n\t"
"movb %1, %b0\n\t"
/* don't need lfence here, because loads are in-order */
"jmp 1b\n"
"2:"
: "+Q" (), "+m" (->)
:
: "memory", "cc");
}
066行:LOCK_PREFIX在UP上为空定义,而在SMP上为Lock,用以保证从066行~074行为原子操作,强制所有CPU缓存失效。xaddw指令用法如下:
xaddw src, dsc ==
tmp = dsc
desc = dsc + src
src = tmp
XADDW语法验证实验:
xaddw使%0和%1按1个word长度交换相加,即:%0: inc → slock, %1: slock → slock + 0x0100。%1此时高字节Next + 1。xaddw使%0和%1内容改变如下:
068行:比较inc中自己的Next是否与Owner中ticket相等,若相等则获取自旋锁使用权、结束循环。
070行 ~ 073行:如果Owner不属于自己,则执行空语句,并重新读取slock中的Owner,跳回至068行进行判断。
为什么要用LOCK_PREFIX宏来代替直接使用lock指令的方式呢?解释如下:为了避免在配置了CONFIG_SMP项编译产生的SMP内核、实际却运行在UP系统上时系统执行lock命令所带来的开销,系统创建在.smp_locks一张SMP alternatives table用以保存系统中所有lock指令的指针。当实际运行时,若从SMP→UP时,可以根据.smp_locks lock 指针表通过热补丁的方式将lock指令替换成nop指令。当然也可以实现系统运行时将锁由UP→SMP的切换。具体应用可参见参考资料《Linux 内核 LOCK_PREFIX 的含义》。
/*
* Alternative inline assembly for SMP.
*
* 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
* 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.
*
* 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
* 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
* and size information. That keeps the table sizes small.
*/
#ifdef CONFIG_SMP
#define \
".section .smp_locks,\"a\"\n" \
".balign 4\n" \
".long 671f - .\n" /* offset */ \
".previous\n" \
"671:"
#define "\n\tlock; "
#else /* ! CONFIG_SMP */
#define ""
#define ""
#endif
032行“.section .smp_locks, a”,表示以下代码生成在.smp_locks段中,而“a”代表——allocatable。
033行~034行 “.balign 4 .long 571f”,表示以4字节对齐、将671标签的地址置于.smp_locks段中,而标签671的地址即为:代码段lock指令的地址。(其实就是lock指令的指针啦~~~)
033行~034行 “.previous”伪指令,表示恢复以前section,即代码段。故在038行将导致在代码段生成lock指令。
时首先尝试使用__ticket_spin_trylock对lock进行加锁,若失败则继续使用__ticket_spin_lock进行加锁。不直接调用__ticket_spin_lock而使用__ticket_spin_trylock的原因是:
trylock首先不会修改lock.slock的ticket,它只是通过再次检查,1)将slock读出,并判断slock是否处于空闲状态;2)调用LOCK执行原子操作,判断当前slock的Next是否已经被其它CPU修改,若未被修改则获得该锁,并将lock.slock.Next + 1。
spin_lock,无论如何,首先调用LOCK执行原子性操作、声明ticket;而trylock则首先进行slock.Next == slock.Owner的判断,降低第二次比较调用LOCK的概率。
static int ( *)
{
int , new;
asm volatile("movzwl %2, %0\n\t"
"cmpb %h0,%b0\n\t"
"leal 0x100(%" "0), %1\n\t"
"jne 1f\n\t"
"cmpxchgw %w1,%2\n\t"
"1:"
"sete %b1\n\t"
"movzbl %b1,%0\n\t"
: "=&a" (), "=&q" (new), "+m" (->)
:
: "memory", "cc");
return ;
}
084行将lock.slock的值赋给tmp。
085行 比较tmp.next == tmp.owner,判断当前自旋锁是否空闲。
086行 leal指令(Load effective address),实际上是movl的变形,“leal 0x10 (%eax, %eax, 3), %edx” → “%edx = 0x10 + %eax + %eax * 3”,但leal却不像movl那样从内存取值、而直接读取寄存器。086行语句,根据REG_PTR_MODE不同配置,在X86平台下为:“leal 0x100(%k0), %1”,而在其它平台为:“leal 0x100(%q0), %1”,忽略占位符修饰“k”或“q”,则该行语句等价于:
“movl (%0 + 0x100),%1”,此时new = { tmp.Next + 1, tmp.Owner }。
087行 若tmp.next != tmp.owner,即自旋锁不空闲,则跳到089行将0赋值给tmp并返回。
088行 原子性地执行操作cmpxchgw,用以检测当前自旋锁是否已被其它CPU修改lock.slock的Next域,若有竞争者则失败、否则获得该锁并将Next + 1,这一系列操作是原子性的!cmpxchgw操作解释如下:
the accumulator (8-32 bits) with "dest". If equal the "dest" is loaded with "src", otherwise the accumulator is loaded with "dest".(在IA32下,%EAX即为累加器。)
所以,“cmpxchgw %w1, %2”等效于:
“tmp.Next == lock.slock.Next ? lock.slock = new : tmp = lock.slock”
若Next未发生变化,则将lock.slock更新为new,实质上是将slock的Next+1。
090行 执行sete指令,若cmpxchgw或cmpb成功则将new的最低字节%b1赋值为1,否则赋值为0. sete的解释为:
Sets the byte in the operand to 1 if the Zero Flag is set, otherwise sets the operand to 0.
091行 movzbl(movz from byte to long)指令将%b1赋值给tmp最低字节,且其它位补0.即将tmp置为0或1.
SMP体系架构-SPIN UNLOCK (ticket_shif 8)
/include/linux/spinlock_api_smp.h
#ifdef CONFIG_INLINE_SPIN_LOCK
#define () ()
#endif
static inline void ( *)
{
(&->, 1, );
();
();
}
spin_unlock即最终调用do_raw_spin_unlock对自旋锁进行释放操作。
/include/linux/spinlock.h
static inline void ( *) ()
{
();
(&->);
}
对于x86的IA32平台,arch_spin_lock实现如下:
/arch/x86/include/asm/spinlock.h
static void ( *)
{
();
}
#if ( < 256)
#define 8
static void ( *)
{
asm volatile( "incb %0"
: "+m" (->)
:
: "memory", "cc");
}
101行 将lock->slock的Owner + 1,表示可以让下一个拥有牌号的CPU加锁。
#if (CONFIG_X86_32) && \
((CONFIG_X86_OOSTORE) || (CONFIG_X86_PPRO_FENCE))
/*
* On PPro SMP or if we are using OOSTORE, we use a locked operation to unlock
* (PPro errata 66, 92)
*/
#
#else
#
#endif
参考资料
自旋锁
《spinlocks.txt》,/Documentation/spinlocks.txt
《Ticket spinlocks》,
《Linux x86 spinlock实现之分析》,
《Linux 内核 LOCK_PREFIX 的含义》,
《The Intel 8086 / 8088/ 80186 / 80286 / 80386 / 80486 Instruction Set》: