# 8.7 安全点分析 ## 回收的安全点 什么时候才能进行抢占呢？如何才能区分该抢占信号是运行时发出的还是用户代码发出的呢？ TODO: TODO: 解释执行栈映射补充寄存器映射，中断信号 SIGURG ``` // wantAsyncPreempt 返回异步抢占是否被 gp 请求 func wantAsyncPreempt(gp *g) bool { // 同时检查 G 和 P return (gp.preempt || gp.m.p != 0 && gp.m.p.ptr().preempt) && readgstatus(gp)&^_Gscan == _Grunning } ``` 什么时候才是安全的异步抢占点呢？ 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 } ``` #### 其他抢占触发点 TODO: 一些 GC 的处理， suspendG preemptStop 会在什么时候被设置为抢占呢？GC。