`ll/sc` 指令在`linux`中的軟件實現

load-link與store-conditional (LL/SC)是一對用於併發同步訪問內存的CPU指令。Load-link返回內存位置處的當前值，隨後的store-conditional在該內存位置處保存新值（若是從load-link後沒有被修改）。這被用於實現無鎖算法與read-modify-write原子操做。

linux在<asm/atomic.h>、<asm/system.h>、<asm/cmpxchg.h>、<asm/bitops.h>、<asm/local.h>中實現了多種基本的原子操做，以最簡單的atomic_add來舉例：

/*
 * atomic_add - add integer to atomic variable
 * @i: integer value to add
 * @v: pointer of type atomic_t
 *
 * Atomically adds @i to @v.
 */
static __inline__ void atomic_add(int i, atomic_t * v)
{
    if (kernel_uses_llsc && R10000_LLSC_WAR) {
        int temp;

        __asm__ __volatile__(
        "   .set    mips3                   \n"
        "1: ll  %0, %1      # atomic_add        \n"
        "   addu    %0, %2                  \n"
        "   sc  %0, %1                  \n"
        "   beqzl   %0, 1b                  \n"
        "   .set    mips0                   \n"
        : "=&r" (temp), "=m" (v->counter)
        : "Ir" (i), "m" (v->counter));
    } else if (kernel_uses_llsc) {
        int temp;

        __asm__ __volatile__(
        "   .set    mips3                   \n"
        "1: ll  %0, %1      # atomic_add        \n"
        "   addu    %0, %2                  \n"
        "   sc  %0, %1                  \n"
        "   beqz    %0, 2f                  \n"
        "   .subsection 2                   \n"
        "2: b   1b                  \n"
        "   .previous                   \n"
        "   .set    mips0                   \n"
        : "=&r" (temp), "=m" (v->counter)
        : "Ir" (i), "m" (v->counter));
    } else {
        unsigned long flags;

        raw_local_irq_save(flags);
        v->counter += i;
        raw_local_irq_restore(flags);
    }
}

其中，kernel_uses_llsc是一個宏定義，當定義爲0時就須要軟件實現。raw_local_irq_save與raw_local_irq_restore函數定義在<linux/irqflags.h>，對於不一樣的mips cpu有不一樣的實現，最簡單的實現以下：

.macro irq_enable_hazard; _ssnop; _ssnop; _ssnop;; .endm
    .macro irq_disable_hazard; nop; nop; nop; .endm

        .macro  raw_local_irq_save result           
    .set    push                        
    .set    reorder                     
    .set    noat                        
    mfc0    \result, $12                    
    ori $1, \result, 0x1f               
    xori    $1, 0x1f                    
    .set    noreorder                   
    mtc0    $1, $12                     
    irq_disable_hazard                  
    .set    pop                     
    .endm                           

        .macro  raw_local_irq_restore flags         
    .set    push                        
    .set    noreorder                   
    .set    noat                        
    mfc0    $1, $12                     
    andi    \flags, 1                   
    ori $1, 0x1f                    
    xori    $1, 0x1f                    
    or  \flags, $1                  
    mtc0    \flags, $12                 
    irq_disable_hazard                  
    .set    pop                     
    .endm

raw_local_irq_save與raw_local_irq_restore被實現爲兩個mips的宏定義，raw_local_irq_save對cp0的status寄存器進行修改，讓cpu進入kernek mode，ERL與EXL置0，同時禁止中斷，從而保證了原子性。raw_local_irq_restore將中斷使能打開。