arm上ldrex和strexeq指令用来尝试获取独占内存权限和设置在独占权限时回写

【duan注】关于LDREX 和 STREX这两对指令的具体使用方法，及其作用（独占访问存储器），请查看 DUI0204IC_rvct_assembler_guide.pdf  手册的148页。 __raw_spin_lock在ARM处理器上的实现/******include/asm-arm/spinlock_types.h***/ typedef struct {        volatile unsigned int lock;        } raw_spinlock_t;#define __RAW_SPIN_LOCK_UNLOCKED { 0 }/******include/asm-arm/spinlock.h***/#if __LINUX_ARM_ARCH__ < 6        #error SMP not supported on pre-ARMv6 CPUs //ARMv6后，才有多核ARM处理器        #endif        ……        static inline void __raw_spin_lock(raw_spinlock_t *lock)        {                unsigned long tmp;                __asm__ __volatile__(        "1: ldrex        %0, [%1]\n"        //取lock->lock放在 tmp里，并且设置&lock->lock这个内存地址为独占访问        "        teq %0, #0\n"        // 测试lock_lock是否为0，影响标志位z        #ifdef CONFIG_CPU_32v6K        "        wfene\n"        #endif        "        strexeq %0, %2, [%1]\n"        //如果lock_lock是0，并且是独占访问这个内存，就向lock->lock里写入1，并向tmp返回0，同时清除独占标记        "        teqeq %0, #0\n"        //如果lock_lock是0，并且strexeq返回了0，表示加锁成功，返回        " bne 1b"        //如果上面的条件(1：lock->lock里不为0，2：strexeq失败)有一个符合，就在原地打转                : "=&r" (tmp) //%0：输出放在tmp里，可以是任意寄存器                : "r" (&lock->lock), "r" (1)         //%1：取&lock->lock放在任意寄存器，%2：任意寄存器放入1                : "cc"); //状态寄存器可能会改变                smp_mb();        }上述代码关键在于LDREX和STREX指令的应用。DREX和STREX指令是在V6以后才出现的，代替了V6以前的 swp指令。可以让bus监控LDREX和STREX指令之间有无其它CPU和DMA来存取过这个地址，若有的话STREX指令的第一个寄存器里设置为 1（动作失败），若没有，指令的第一个寄存器里设置为0（动作成功）。不仅是自旋锁用到LDREX和STREX指令，信号量的实现也是利用LDREX和STREX指令来实现的。

文件：Spinlock_types.h (\kernel\arch\arm\include\asm)#define TICKET_SHIFT<span style="white-space:pre"></span>16  //在lock时，用到它来讲0x100加到slock上，也即加到next成员上typedef struct {<span style="white-space:pre"></span>union {<span style="white-space:pre"></span>u32 slock;  //当spin lock成功后，slock的高16位的值比低16位值大1，否则两部分的的值相等<span style="white-space:pre"></span>struct __raw_tickets { #ifdef __ARMEB__<span style="white-space:pre"></span>u16 next;<span style="white-space:pre"></span>u16 owner;#else<span style="white-space:pre"></span>u16 owner;  //与slock成员构成了一个union体。owner是低地址<span style="white-space:pre"></span>u16 next;#endif<span style="white-space:pre"></span>} tickets;<span style="white-space:pre"></span>};} arch_spinlock_t;文件：Spinlock.h (\kernel\arch\arm\include\asm)//枷锁static inline void arch_spin_lock(arch_spinlock_t *lock)  {<span style="white-space:pre"></span>unsigned long tmp, flags = 0;<span style="white-space:pre"></span>u32 newval;<span style="white-space:pre"></span>arch_spinlock_t lockval;<span style="white-space:pre"></span>//从以下可以看出，当spin lock成功后，slock的高16位的值比低16位值大1.<span style="white-space:pre"></span>__asm__ __volatile__("1:<span style="white-space:pre"></span>ldrex<span style="white-space:pre"></span>%0, [%3]\n"   //lockval=lock->slock"<span style="white-space:pre"></span>add<span style="white-space:pre"></span>%1, %0, %4\n"    //newval=lockval+0x100"<span style="white-space:pre"></span>strex<span style="white-space:pre"></span>%2, %1, [%3]\n" //lock->slock=newval;如果LDREX和STREX指令之间无其它CPU和DMA来存取过这个地址&lock->slock，则STREX返回0表示枷锁成功（否则返回1），返回值放到tmp中"<span style="white-space:pre"></span>teq<span style="white-space:pre"></span>%2, #0\n"  //检查tmp的值"<span style="white-space:pre"></span>bne<span style="white-space:pre"></span>1b"<span style="white-space:pre"></span>: "=&r" (lockval), "=&r" (newval), "=&r" (tmp)<span style="white-space:pre"></span>: "r" (&lock->slock), "I" (1 << TICKET_SHIFT)<span style="white-space:pre"></span>: "cc");<span style="white-space:pre"></span>while (lockval.tickets.next != lockval.tickets.owner) {<span style="white-space:pre"></span>if (msm_krait_need_wfe_fixup) {  //不满足<span style="white-space:pre"></span>local_save_flags(flags);<span style="white-space:pre"></span>local_fiq_disable();<span style="white-space:pre"></span>__asm__ __volatile__(<span style="white-space:pre"></span>"mrc<span style="white-space:pre"></span>p15, 7, %0, c15, c0, 5\n"<span style="white-space:pre"></span>: "=r" (tmp)<span style="white-space:pre"></span>:<span style="white-space:pre"></span>: "cc");<span style="white-space:pre"></span>tmp &= ~(0x10000);<span style="white-space:pre"></span>__asm__ __volatile__(<span style="white-space:pre"></span>"mcr<span style="white-space:pre"></span>p15, 7, %0, c15, c0, 5\n"<span style="white-space:pre"></span>:<span style="white-space:pre"></span>: "r" (tmp)<span style="white-space:pre"></span>: "cc");<span style="white-space:pre"></span>isb();<span style="white-space:pre"></span>}<span style="white-space:pre"></span>wfe();<span style="white-space:pre"></span>if (msm_krait_need_wfe_fixup) {<span style="white-space:pre"></span>tmp |= 0x10000;<span style="white-space:pre"></span>__asm__ __volatile__(<span style="white-space:pre"></span>"mcr<span style="white-space:pre"></span>p15, 7, %0, c15, c0, 5\n"<span style="white-space:pre"></span>:<span style="white-space:pre"></span>: "r" (tmp)<span style="white-space:pre"></span>: "cc");<span style="white-space:pre"></span>isb();<span style="white-space:pre"></span>local_irq_restore(flags);<span style="white-space:pre"></span>}<span style="white-space:pre"></span>lockval.tickets.owner = ACCESS_ONCE(lock->tickets.owner);<span style="white-space:pre"></span>}<span style="white-space:pre"></span>smp_mb();}//解锁static inline void arch_spin_unlock(arch_spinlock_t *lock){<span style="white-space:pre"></span>smp_mb();<span style="white-space:pre"></span>lock->tickets.owner++;  //owner++使得它与next的值相等<span style="white-space:pre"></span>dsb_sev();}