# 8.7 安全點分析 ## 回收的安全點 什么時候才能進行搶占呢？如何才能區分該搶占信號是運行時發出的還是用戶代碼發出的呢？ TODO: TODO: 解釋執行棧映射補充寄存器映射，中斷信號 SIGURG ``` // wantAsyncPreempt 返回異步搶占是否被 gp 請求 func wantAsyncPreempt(gp *g) bool { // 同時檢查 G 和 P return (gp.preempt || gp.m.p != 0 && gp.m.p.ptr().preempt) && readgstatus(gp)&^_Gscan == _Grunning } ``` 什么時候才是安全的異步搶占點呢？ TODO: ``` func isAsyncSafePoint(gp *g, pc, sp, lr uintptr) bool { mp := gp.m // Only user Gs can have safe-points. We check this first // because it's extremely common that we'll catch mp in the // scheduler processing this G preemption. if mp.curg != gp { return false } // Check M state. if mp.p == 0|| !canPreemptM(mp) { return false } // Check stack space. if sp < gp.stack.lo || sp-gp.stack.lo < asyncPreemptStack { return false } // Check if PC is an unsafe-point. f := findfunc(pc) if !f.valid() { // Not Go code. return false } ... smi := pcdatavalue(f, _PCDATA_RegMapIndex, pc, nil) if smi == -2 { // Unsafe-point marked by compiler. This includes // atomic sequences (e.g., write barrier) and nosplit // functions (except at calls). return false } if fd := funcdata(f, _FUNCDATA_LocalsPointerMaps); fd == nil || fd == unsafe.Pointer(&no_pointers_stackmap) { // This is assembly code. Don't assume it's // well-formed. We identify assembly code by // checking that it has either no stack map, or // no_pointers_stackmap, which is the stack map // for ones marked as NO_LOCAL_POINTERS. // // TODO: Are there cases that are safe but don't have a // locals pointer map, like empty frame functions? return false } if hasPrefix(funcname(f), "runtime.") || hasPrefix(funcname(f), "runtime/internal/") || hasPrefix(funcname(f), "reflect.") { // For now we never async preempt the runtime or // anything closely tied to the runtime. Known issues // include: various points in the scheduler ("don't // preempt between here and here"), much of the defer // implementation (untyped info on stack), bulk write // barriers (write barrier check), // reflect.{makeFuncStub,methodValueCall}. // // TODO(austin): We should improve this, or opt things // in incrementally. return false } return true } ``` #### 其他搶占觸發點 TODO: 一些 GC 的處理， suspendG preemptStop 會在什么時候被設置為搶占呢？GC。