1 // Copyright 2015 The Go Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style 3 // license that can be found in the LICENSE file. 4 5 // Garbage collector: write barriers. 6 // 7 // For the concurrent garbage collector, the Go compiler implements 8 // updates to pointer-valued fields that may be in heap objects by 9 // emitting calls to write barriers. The main write barrier for 10 // individual pointer writes is gcWriteBarrier and is implemented in 11 // assembly. This file contains write barrier entry points for bulk 12 // operations. See also mwbbuf.go. 13 14 package runtime 15 16 import ( 17 "internal/abi" 18 "runtime/internal/sys" 19 "unsafe" 20 ) 21 22 // Go uses a hybrid barrier that combines a Yuasa-style deletion 23 // barrier—which shades the object whose reference is being 24 // overwritten—with Dijkstra insertion barrier—which shades the object 25 // whose reference is being written. The insertion part of the barrier 26 // is necessary while the calling goroutine's stack is grey. In 27 // pseudocode, the barrier is: 28 // 29 // writePointer(slot, ptr): 30 // shade(*slot) 31 // if current stack is grey: 32 // shade(ptr) 33 // *slot = ptr 34 // 35 // slot is the destination in Go code. 36 // ptr is the value that goes into the slot in Go code. 37 // 38 // Shade indicates that it has seen a white pointer by adding the referent 39 // to wbuf as well as marking it. 40 // 41 // The two shades and the condition work together to prevent a mutator 42 // from hiding an object from the garbage collector: 43 // 44 // 1. shade(*slot) prevents a mutator from hiding an object by moving 45 // the sole pointer to it from the heap to its stack. If it attempts 46 // to unlink an object from the heap, this will shade it. 47 // 48 // 2. shade(ptr) prevents a mutator from hiding an object by moving 49 // the sole pointer to it from its stack into a black object in the 50 // heap. If it attempts to install the pointer into a black object, 51 // this will shade it. 52 // 53 // 3. Once a goroutine's stack is black, the shade(ptr) becomes 54 // unnecessary. shade(ptr) prevents hiding an object by moving it from 55 // the stack to the heap, but this requires first having a pointer 56 // hidden on the stack. Immediately after a stack is scanned, it only 57 // points to shaded objects, so it's not hiding anything, and the 58 // shade(*slot) prevents it from hiding any other pointers on its 59 // stack. 60 // 61 // For a detailed description of this barrier and proof of 62 // correctness, see https://github.com/golang/proposal/blob/master/design/17503-eliminate-rescan.md 63 // 64 // 65 // 66 // Dealing with memory ordering: 67 // 68 // Both the Yuasa and Dijkstra barriers can be made conditional on the 69 // color of the object containing the slot. We chose not to make these 70 // conditional because the cost of ensuring that the object holding 71 // the slot doesn't concurrently change color without the mutator 72 // noticing seems prohibitive. 73 // 74 // Consider the following example where the mutator writes into 75 // a slot and then loads the slot's mark bit while the GC thread 76 // writes to the slot's mark bit and then as part of scanning reads 77 // the slot. 78 // 79 // Initially both [slot] and [slotmark] are 0 (nil) 80 // Mutator thread GC thread 81 // st [slot], ptr st [slotmark], 1 82 // 83 // ld r1, [slotmark] ld r2, [slot] 84 // 85 // Without an expensive memory barrier between the st and the ld, the final 86 // result on most HW (including 386/amd64) can be r1==r2==0. This is a classic 87 // example of what can happen when loads are allowed to be reordered with older 88 // stores (avoiding such reorderings lies at the heart of the classic 89 // Peterson/Dekker algorithms for mutual exclusion). Rather than require memory 90 // barriers, which will slow down both the mutator and the GC, we always grey 91 // the ptr object regardless of the slot's color. 92 // 93 // Another place where we intentionally omit memory barriers is when 94 // accessing mheap_.arena_used to check if a pointer points into the 95 // heap. On relaxed memory machines, it's possible for a mutator to 96 // extend the size of the heap by updating arena_used, allocate an 97 // object from this new region, and publish a pointer to that object, 98 // but for tracing running on another processor to observe the pointer 99 // but use the old value of arena_used. In this case, tracing will not 100 // mark the object, even though it's reachable. However, the mutator 101 // is guaranteed to execute a write barrier when it publishes the 102 // pointer, so it will take care of marking the object. A general 103 // consequence of this is that the garbage collector may cache the 104 // value of mheap_.arena_used. (See issue #9984.) 105 // 106 // 107 // Stack writes: 108 // 109 // The compiler omits write barriers for writes to the current frame, 110 // but if a stack pointer has been passed down the call stack, the 111 // compiler will generate a write barrier for writes through that 112 // pointer (because it doesn't know it's not a heap pointer). 113 // 114 // One might be tempted to ignore the write barrier if slot points 115 // into to the stack. Don't do it! Mark termination only re-scans 116 // frames that have potentially been active since the concurrent scan, 117 // so it depends on write barriers to track changes to pointers in 118 // stack frames that have not been active. 119 // 120 // 121 // Global writes: 122 // 123 // The Go garbage collector requires write barriers when heap pointers 124 // are stored in globals. Many garbage collectors ignore writes to 125 // globals and instead pick up global -> heap pointers during 126 // termination. This increases pause time, so we instead rely on write 127 // barriers for writes to globals so that we don't have to rescan 128 // global during mark termination. 129 // 130 // 131 // Publication ordering: 132 // 133 // The write barrier is *pre-publication*, meaning that the write 134 // barrier happens prior to the *slot = ptr write that may make ptr 135 // reachable by some goroutine that currently cannot reach it. 136 // 137 // 138 // Signal handler pointer writes: 139 // 140 // In general, the signal handler cannot safely invoke the write 141 // barrier because it may run without a P or even during the write 142 // barrier. 143 // 144 // There is exactly one exception: profbuf.go omits a barrier during 145 // signal handler profile logging. That's safe only because of the 146 // deletion barrier. See profbuf.go for a detailed argument. If we 147 // remove the deletion barrier, we'll have to work out a new way to 148 // handle the profile logging. 149 150 // typedmemmove copies a value of type t to dst from src. 151 // Must be nosplit, see #16026. 152 // 153 // TODO: Perfect for go:nosplitrec since we can't have a safe point 154 // anywhere in the bulk barrier or memmove. 155 // 156 //go:nosplit 157 func typedmemmove(typ *_type, dst, src unsafe.Pointer) { 158 if dst == src { 159 return 160 } 161 if writeBarrier.needed && typ.ptrdata != 0 { 162 bulkBarrierPreWrite(uintptr(dst), uintptr(src), typ.ptrdata) 163 } 164 // There's a race here: if some other goroutine can write to 165 // src, it may change some pointer in src after we've 166 // performed the write barrier but before we perform the 167 // memory copy. This safe because the write performed by that 168 // other goroutine must also be accompanied by a write 169 // barrier, so at worst we've unnecessarily greyed the old 170 // pointer that was in src. 171 memmove(dst, src, typ.size) 172 if writeBarrier.cgo { 173 cgoCheckMemmove(typ, dst, src, 0, typ.size) 174 } 175 } 176 177 //go:linkname reflect_typedmemmove reflect.typedmemmove 178 func reflect_typedmemmove(typ *_type, dst, src unsafe.Pointer) { 179 if raceenabled { 180 raceWriteObjectPC(typ, dst, getcallerpc(), funcPC(reflect_typedmemmove)) 181 raceReadObjectPC(typ, src, getcallerpc(), funcPC(reflect_typedmemmove)) 182 } 183 if msanenabled { 184 msanwrite(dst, typ.size) 185 msanread(src, typ.size) 186 } 187 typedmemmove(typ, dst, src) 188 } 189 190 //go:linkname reflectlite_typedmemmove internal/reflectlite.typedmemmove 191 func reflectlite_typedmemmove(typ *_type, dst, src unsafe.Pointer) { 192 reflect_typedmemmove(typ, dst, src) 193 } 194 195 // typedmemmovepartial is like typedmemmove but assumes that 196 // dst and src point off bytes into the value and only copies size bytes. 197 // off must be a multiple of sys.PtrSize. 198 //go:linkname reflect_typedmemmovepartial reflect.typedmemmovepartial 199 func reflect_typedmemmovepartial(typ *_type, dst, src unsafe.Pointer, off, size uintptr) { 200 if writeBarrier.needed && typ.ptrdata > off && size >= sys.PtrSize { 201 if off&(sys.PtrSize-1) != 0 { 202 panic("reflect: internal error: misaligned offset") 203 } 204 pwsize := alignDown(size, sys.PtrSize) 205 if poff := typ.ptrdata - off; pwsize > poff { 206 pwsize = poff 207 } 208 bulkBarrierPreWrite(uintptr(dst), uintptr(src), pwsize) 209 } 210 211 memmove(dst, src, size) 212 if writeBarrier.cgo { 213 cgoCheckMemmove(typ, dst, src, off, size) 214 } 215 } 216 217 // reflectcallmove is invoked by reflectcall to copy the return values 218 // out of the stack and into the heap, invoking the necessary write 219 // barriers. dst, src, and size describe the return value area to 220 // copy. typ describes the entire frame (not just the return values). 221 // typ may be nil, which indicates write barriers are not needed. 222 // 223 // It must be nosplit and must only call nosplit functions because the 224 // stack map of reflectcall is wrong. 225 // 226 //go:nosplit 227 func reflectcallmove(typ *_type, dst, src unsafe.Pointer, size uintptr, regs *abi.RegArgs) { 228 if writeBarrier.needed && typ != nil && typ.ptrdata != 0 && size >= sys.PtrSize { 229 bulkBarrierPreWrite(uintptr(dst), uintptr(src), size) 230 } 231 memmove(dst, src, size) 232 233 // Move pointers returned in registers to a place where the GC can see them. 234 for i := range regs.Ints { 235 if regs.ReturnIsPtr.Get(i) { 236 regs.Ptrs[i] = unsafe.Pointer(regs.Ints[i]) 237 } 238 } 239 } 240 241 //go:nosplit 242 func typedslicecopy(typ *_type, dstPtr unsafe.Pointer, dstLen int, srcPtr unsafe.Pointer, srcLen int) int { 243 n := dstLen 244 if n > srcLen { 245 n = srcLen 246 } 247 if n == 0 { 248 return 0 249 } 250 251 // The compiler emits calls to typedslicecopy before 252 // instrumentation runs, so unlike the other copying and 253 // assignment operations, it's not instrumented in the calling 254 // code and needs its own instrumentation. 255 if raceenabled { 256 callerpc := getcallerpc() 257 pc := funcPC(slicecopy) 258 racewriterangepc(dstPtr, uintptr(n)*typ.size, callerpc, pc) 259 racereadrangepc(srcPtr, uintptr(n)*typ.size, callerpc, pc) 260 } 261 if msanenabled { 262 msanwrite(dstPtr, uintptr(n)*typ.size) 263 msanread(srcPtr, uintptr(n)*typ.size) 264 } 265 266 if writeBarrier.cgo { 267 cgoCheckSliceCopy(typ, dstPtr, srcPtr, n) 268 } 269 270 if dstPtr == srcPtr { 271 return n 272 } 273 274 // Note: No point in checking typ.ptrdata here: 275 // compiler only emits calls to typedslicecopy for types with pointers, 276 // and growslice and reflect_typedslicecopy check for pointers 277 // before calling typedslicecopy. 278 size := uintptr(n) * typ.size 279 if writeBarrier.needed { 280 pwsize := size - typ.size + typ.ptrdata 281 bulkBarrierPreWrite(uintptr(dstPtr), uintptr(srcPtr), pwsize) 282 } 283 // See typedmemmove for a discussion of the race between the 284 // barrier and memmove. 285 memmove(dstPtr, srcPtr, size) 286 return n 287 } 288 289 //go:linkname reflect_typedslicecopy reflect.typedslicecopy 290 func reflect_typedslicecopy(elemType *_type, dst, src slice) int { 291 if elemType.ptrdata == 0 { 292 return slicecopy(dst.array, dst.len, src.array, src.len, elemType.size) 293 } 294 return typedslicecopy(elemType, dst.array, dst.len, src.array, src.len) 295 } 296 297 // typedmemclr clears the typed memory at ptr with type typ. The 298 // memory at ptr must already be initialized (and hence in type-safe 299 // state). If the memory is being initialized for the first time, see 300 // memclrNoHeapPointers. 301 // 302 // If the caller knows that typ has pointers, it can alternatively 303 // call memclrHasPointers. 304 // 305 //go:nosplit 306 func typedmemclr(typ *_type, ptr unsafe.Pointer) { 307 if writeBarrier.needed && typ.ptrdata != 0 { 308 bulkBarrierPreWrite(uintptr(ptr), 0, typ.ptrdata) 309 } 310 memclrNoHeapPointers(ptr, typ.size) 311 } 312 313 //go:linkname reflect_typedmemclr reflect.typedmemclr 314 func reflect_typedmemclr(typ *_type, ptr unsafe.Pointer) { 315 typedmemclr(typ, ptr) 316 } 317 318 //go:linkname reflect_typedmemclrpartial reflect.typedmemclrpartial 319 func reflect_typedmemclrpartial(typ *_type, ptr unsafe.Pointer, off, size uintptr) { 320 if writeBarrier.needed && typ.ptrdata != 0 { 321 bulkBarrierPreWrite(uintptr(ptr), 0, size) 322 } 323 memclrNoHeapPointers(ptr, size) 324 } 325 326 // memclrHasPointers clears n bytes of typed memory starting at ptr. 327 // The caller must ensure that the type of the object at ptr has 328 // pointers, usually by checking typ.ptrdata. However, ptr 329 // does not have to point to the start of the allocation. 330 // 331 //go:nosplit 332 func memclrHasPointers(ptr unsafe.Pointer, n uintptr) { 333 bulkBarrierPreWrite(uintptr(ptr), 0, n) 334 memclrNoHeapPointers(ptr, n) 335 } 336