函数arch_spin_lock()实现：
static inline void arch_spin_lock(arch_spinlock_t *lock)
{
unsigned int tmp;
arch_spinlock_t lockval, newval;
asm volatile(
/* Atomically increment the next ticket. */
ARM64_LSE_ATOMIC_INSN(
/* LL/SC */
" prfm pstl1strm, %3\n"
"1: ldaxr %w0, %3\n"
" add %w1, %w0, %w5\n"
" stxr %w2, %w1, %3\n"
" cbnz %w2, 1b\n",
/* LSE atomics */
" mov %w2, %w5\n"
" ldadda %w2, %w0, %3\n"
__nops(3)
)
/* Did we get the lock? */
" eor %w1, %w0, %w0, ror #16\n"
" cbz %w1, 3f\n"
/*
* No: spin on the owner. Send a local event to avoid missing an
* unlock before the exclusive load.
*/
" sevl\n"
"2: wfe\n"
" ldaxrh %w2, %4\n"
" eor %w1, %w2, %w0, lsr #16\n"
" cbnz %w1, 2b\n"
/* We got the lock. Critical section starts here. */
"3:"
: "=&r" (lockval), "=&r" (newval), "=&r" (tmp), "+Q" (*lock)
: "Q" (lock->owner), "I" (1 << TICKET_SHIFT)
: "memory");
}
在理解上面操作之前，先看看ARMv8手册相关说明：
A PE can use the Wait for Event (WFE) mechanism to enter a low-power state, depending on the value of an Event
Register for that PE. To enter the low-power state, the PE executes a Wait For Event instruction, WFE, and if the Event
Register is clear, the PE can enter the low-power state.
If the PE does enter the low-power state, it remains in that low-power state until it receives a WFE wake-up event.
The architecture does not define the exact nature of the low-power state, except that the execution of a WFE
instruction must not cause a loss of memory coherency.
WFE mechanism behavior depends on the interaction of all of the following, that are described in the subsections
that follow:
• The Event Register for the PE. See subsection The Event Register on page D1-1529.
• The Wait For Event instruction, WFE. See subsection The Wait For Event instruction on page D1-1529.
• WFE wake-up events. See subsection WFE wake-up events in AArch64 state on page D1-1530
• The Send Event instructions, SEV and SEVL that can cause WFE wake-up events. See subsection The Send
Event instructions on page D1-1530.
也就是说，PE可以进入低功耗模式，采用执行WFE指令。WFE指令实现基于一个Event register，即一个事件寄存器，如果事件寄存器被清除
则PE进入低功耗，进而执行WFE。这意味着当Event register被设置时，PE被唤醒。当然，这种是处理器内部逻辑来唤醒的。
还需要注意的是，这里的WFE指令到底是让处理器干啥，是未定义的，只要不让RAM丢失数据即可。也就是维护好cache 和memory一致性
就好。
WFE依赖下面几点：
1. 一个Event Register寄存器。
2. 一个等待Event的指令，即WFE。
3. WFE所等待的唤醒事件，定义哪些事件可以唤醒。
4. 发送事件指令，即WEV和SEVL。这两个指令可以触发唤醒事件，进而让处于WFE状态的PE重新工作起来。
详细的内容读者可以参考上面给出的文档链接。
所以，我们可以看到函数arch_spin_lock()实现。其中有三个标号，代表了三个跳转。标号3代表获取了锁，函数执行到此后，简单设置后返回。
标号2到标号3是一个WFE状态检查，即处理唤醒事件，如果么有唤醒事件，则继续处于WFE状态。注意标号2之前的SEVL指令，其用于发送唤醒事件。
可以想象，SEVL是个处理器间的唤醒发送操作。我们这段spin_lock代码，真正发送事件的应该是在unlock中。
static inline int arch_spin_trylock(arch_spinlock_t *lock)
{
unsigned int tmp;
arch_spinlock_t lockval;
asm volatile(ARM64_LSE_ATOMIC_INSN(
/* LL/SC */
" prfm pstl1strm, %2\n"
"1: ldaxr %w0, %2\n"
" eor %w1, %w0, %w0, ror #16\n"
" cbnz %w1, 2f\n"
" add %w0, %w0, %3\n"
" stxr %w1, %w0, %2\n"
" cbnz %w1, 1b\n"
"2:",
/* LSE atomics */
" ldr %w0, %2\n"
" eor %w1, %w0, %w0, ror #16\n"
" cbnz %w1, 1f\n"
" add %w1, %w0, %3\n"
" casa %w0, %w1, %2\n"
" and %w1, %w1, #0xffff\n"
" eor %w1, %w1, %w0, lsr #16\n"
"1:")
: "=&r" (lockval), "=&r" (tmp), "+Q" (*lock)
: "I" (1 << TICKET_SHIFT)
: "memory");
return !tmp;
}
static inline void arch_spin_unlock(arch_spinlock_t *lock)
{
unsigned long tmp;
asm volatile(ARM64_LSE_ATOMIC_INSN(
/* LL/SC */
" ldrh %w1, %0\n"
" add %w1, %w1, #1\n"
" stlrh %w1, %0",
/* LSE atomics */
" mov %w1, #1\n"
" staddlh %w1, %0\n"
__nops(1))
: "=Q" (lock->owner), "=&r" (tmp)
:
: "memory");
}
/*
* Write lock implementation.
*
* Write locks set bit 31. Unlocking, is done by writing 0 since the lock is
* exclusively held.
*
* The memory barriers are implicit with the load-acquire and store-release
* instructions.
*/
static inline void arch_write_lock(arch_rwlock_t *rw)
{
unsigned int tmp;
asm volatile(ARM64_LSE_ATOMIC_INSN(
/* LL/SC */
" sevl\n"
"1: wfe\n"
"2: ldaxr %w0, %1\n"
" cbnz %w0, 1b\n"
" stxr %w0, %w2, %1\n"
" cbnz %w0, 2b\n"
__nops(1),
/* LSE atomics */
"1: mov %w0, wzr\n"
"2: casa %w0, %w2, %1\n"
" cbz %w0, 3f\n"
" ldxr %w0, %1\n"
" cbz %w0, 2b\n"
" wfe\n"
" b 1b\n"
"3:")
: "=&r" (tmp), "+Q" (rw->lock)
: "r" (0x80000000)
: "memory");
}
static inline int arch_write_trylock(arch_rwlock_t *rw)
{
unsigned int tmp;
asm volatile(ARM64_LSE_ATOMIC_INSN(
/* LL/SC */
"1: ldaxr %w0, %1\n"
" cbnz %w0, 2f\n"
" stxr %w0, %w2, %1\n"
" cbnz %w0, 1b\n"
"2:",
/* LSE atomics */
" mov %w0, wzr\n"
" casa %w0, %w2, %1\n"
__nops(2))
: "=&r" (tmp), "+Q" (rw->lock)
: "r" (0x80000000)
: "memory");
return !tmp;
}
static inline void arch_write_unlock(arch_rwlock_t *rw)
{
asm volatile(ARM64_LSE_ATOMIC_INSN(
" stlr wzr, %0",
" swpl wzr, wzr, %0")
: "=Q" (rw->lock) :: "memory");
}
/*
* Read lock implementation.
*
* It exclusively loads the lock value, increments it and stores the new value
* back if positive and the CPU still exclusively owns the location. If the
* value is negative, the lock is already held.
*
* During unlocking there may be multiple active read locks but no write lock.
*
* The memory barriers are implicit with the load-acquire and store-release
* instructions.
*
* Note that in UNDEFINED cases, such as unlocking a lock twice, the LL/SC
* and LSE implementations may exhibit different behaviour (although this
* will have no effect on lockdep).
*/
static inline void arch_read_lock(arch_rwlock_t *rw)
{
unsigned int tmp, tmp2;
asm volatile(
" sevl\n"
ARM64_LSE_ATOMIC_INSN(
/* LL/SC */
"1: wfe\n"
"2: ldaxr %w0, %2\n"
" add %w0, %w0, #1\n"
" tbnz %w0, #31, 1b\n"
" stxr %w1, %w0, %2\n"
" cbnz %w1, 2b\n"
__nops(1),
/* LSE atomics */
"1: wfe\n"
"2: ldxr %w0, %2\n"
" adds %w1, %w0, #1\n"
" tbnz %w1, #31, 1b\n"
" casa %w0, %w1, %2\n"
" sbc %w0, %w1, %w0\n"
" cbnz %w0, 2b")
: "=&r" (tmp), "=&r" (tmp2), "+Q" (rw->lock)
:
: "cc", "memory");
}
static inline void arch_read_unlock(arch_rwlock_t *rw)
{
unsigned int tmp, tmp2;
asm volatile(ARM64_LSE_ATOMIC_INSN(
/* LL/SC */
"1: ldxr %w0, %2\n"
" sub %w0, %w0, #1\n"
" stlxr %w1, %w0, %2\n"
" cbnz %w1, 1b",
/* LSE atomics */
" movn %w0, #0\n"
" staddl %w0, %2\n"
__nops(2))
: "=&r" (tmp), "=&r" (tmp2), "+Q" (rw->lock)
:
: "memory");
}
static inline int arch_read_trylock(arch_rwlock_t *rw)
{
unsigned int tmp, tmp2;
asm volatile(ARM64_LSE_ATOMIC_INSN(
/* LL/SC */
" mov %w1, #1\n"
"1: ldaxr %w0, %2\n"
" add %w0, %w0, #1\n"
" tbnz %w0, #31, 2f\n"
" stxr %w1, %w0, %2\n"
" cbnz %w1, 1b\n"
"2:",
/* LSE atomics */
" ldr %w0, %2\n"
" adds %w1, %w0, #1\n"
" tbnz %w1, #31, 1f\n"
" casa %w0, %w1, %2\n"
" sbc %w1, %w1, %w0\n"
__nops(1)
"1:")
: "=&r" (tmp), "=&r" (tmp2), "+Q" (rw->lock)
:
: "cc", "memory");
return !tmp2;
}
