Source file src/runtime/mfinal.go
Documentation: runtime
1 // Copyright 2009 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: finalizers and block profiling. 6 7 package runtime 8 9 import ( 10 "internal/abi" 11 "runtime/internal/atomic" 12 "runtime/internal/sys" 13 "unsafe" 14 ) 15 16 // finblock is an array of finalizers to be executed. finblocks are 17 // arranged in a linked list for the finalizer queue. 18 // 19 // finblock is allocated from non-GC'd memory, so any heap pointers 20 // must be specially handled. GC currently assumes that the finalizer 21 // queue does not grow during marking (but it can shrink). 22 // 23 //go:notinheap 24 type finblock struct { 25 alllink *finblock 26 next *finblock 27 cnt uint32 28 _ int32 29 fin [(_FinBlockSize - 2*sys.PtrSize - 2*4) / unsafe.Sizeof(finalizer{})]finalizer 30 } 31 32 var finlock mutex // protects the following variables 33 var fing *g // goroutine that runs finalizers 34 var finq *finblock // list of finalizers that are to be executed 35 var finc *finblock // cache of free blocks 36 var finptrmask [_FinBlockSize / sys.PtrSize / 8]byte 37 var fingwait bool 38 var fingwake bool 39 var allfin *finblock // list of all blocks 40 41 // NOTE: Layout known to queuefinalizer. 42 type finalizer struct { 43 fn *funcval // function to call (may be a heap pointer) 44 arg unsafe.Pointer // ptr to object (may be a heap pointer) 45 nret uintptr // bytes of return values from fn 46 fint *_type // type of first argument of fn 47 ot *ptrtype // type of ptr to object (may be a heap pointer) 48 } 49 50 var finalizer1 = [...]byte{ 51 // Each Finalizer is 5 words, ptr ptr INT ptr ptr (INT = uintptr here) 52 // Each byte describes 8 words. 53 // Need 8 Finalizers described by 5 bytes before pattern repeats: 54 // ptr ptr INT ptr ptr 55 // ptr ptr INT ptr ptr 56 // ptr ptr INT ptr ptr 57 // ptr ptr INT ptr ptr 58 // ptr ptr INT ptr ptr 59 // ptr ptr INT ptr ptr 60 // ptr ptr INT ptr ptr 61 // ptr ptr INT ptr ptr 62 // aka 63 // 64 // ptr ptr INT ptr ptr ptr ptr INT 65 // ptr ptr ptr ptr INT ptr ptr ptr 66 // ptr INT ptr ptr ptr ptr INT ptr 67 // ptr ptr ptr INT ptr ptr ptr ptr 68 // INT ptr ptr ptr ptr INT ptr ptr 69 // 70 // Assumptions about Finalizer layout checked below. 71 1<<0 | 1<<1 | 0<<2 | 1<<3 | 1<<4 | 1<<5 | 1<<6 | 0<<7, 72 1<<0 | 1<<1 | 1<<2 | 1<<3 | 0<<4 | 1<<5 | 1<<6 | 1<<7, 73 1<<0 | 0<<1 | 1<<2 | 1<<3 | 1<<4 | 1<<5 | 0<<6 | 1<<7, 74 1<<0 | 1<<1 | 1<<2 | 0<<3 | 1<<4 | 1<<5 | 1<<6 | 1<<7, 75 0<<0 | 1<<1 | 1<<2 | 1<<3 | 1<<4 | 0<<5 | 1<<6 | 1<<7, 76 } 77 78 func queuefinalizer(p unsafe.Pointer, fn *funcval, nret uintptr, fint *_type, ot *ptrtype) { 79 if gcphase != _GCoff { 80 // Currently we assume that the finalizer queue won't 81 // grow during marking so we don't have to rescan it 82 // during mark termination. If we ever need to lift 83 // this assumption, we can do it by adding the 84 // necessary barriers to queuefinalizer (which it may 85 // have automatically). 86 throw("queuefinalizer during GC") 87 } 88 89 lock(&finlock) 90 if finq == nil || finq.cnt == uint32(len(finq.fin)) { 91 if finc == nil { 92 finc = (*finblock)(persistentalloc(_FinBlockSize, 0, &memstats.gcMiscSys)) 93 finc.alllink = allfin 94 allfin = finc 95 if finptrmask[0] == 0 { 96 // Build pointer mask for Finalizer array in block. 97 // Check assumptions made in finalizer1 array above. 98 if (unsafe.Sizeof(finalizer{}) != 5*sys.PtrSize || 99 unsafe.Offsetof(finalizer{}.fn) != 0 || 100 unsafe.Offsetof(finalizer{}.arg) != sys.PtrSize || 101 unsafe.Offsetof(finalizer{}.nret) != 2*sys.PtrSize || 102 unsafe.Offsetof(finalizer{}.fint) != 3*sys.PtrSize || 103 unsafe.Offsetof(finalizer{}.ot) != 4*sys.PtrSize) { 104 throw("finalizer out of sync") 105 } 106 for i := range finptrmask { 107 finptrmask[i] = finalizer1[i%len(finalizer1)] 108 } 109 } 110 } 111 block := finc 112 finc = block.next 113 block.next = finq 114 finq = block 115 } 116 f := &finq.fin[finq.cnt] 117 atomic.Xadd(&finq.cnt, +1) // Sync with markroots 118 f.fn = fn 119 f.nret = nret 120 f.fint = fint 121 f.ot = ot 122 f.arg = p 123 fingwake = true 124 unlock(&finlock) 125 } 126 127 //go:nowritebarrier 128 func iterate_finq(callback func(*funcval, unsafe.Pointer, uintptr, *_type, *ptrtype)) { 129 for fb := allfin; fb != nil; fb = fb.alllink { 130 for i := uint32(0); i < fb.cnt; i++ { 131 f := &fb.fin[i] 132 callback(f.fn, f.arg, f.nret, f.fint, f.ot) 133 } 134 } 135 } 136 137 func wakefing() *g { 138 var res *g 139 lock(&finlock) 140 if fingwait && fingwake { 141 fingwait = false 142 fingwake = false 143 res = fing 144 } 145 unlock(&finlock) 146 return res 147 } 148 149 var ( 150 fingCreate uint32 151 fingRunning bool 152 ) 153 154 func createfing() { 155 // start the finalizer goroutine exactly once 156 if fingCreate == 0 && atomic.Cas(&fingCreate, 0, 1) { 157 go runfinq() 158 } 159 } 160 161 // This is the goroutine that runs all of the finalizers 162 func runfinq() { 163 var ( 164 frame unsafe.Pointer 165 framecap uintptr 166 argRegs int 167 ) 168 169 for { 170 lock(&finlock) 171 fb := finq 172 finq = nil 173 if fb == nil { 174 gp := getg() 175 fing = gp 176 fingwait = true 177 goparkunlock(&finlock, waitReasonFinalizerWait, traceEvGoBlock, 1) 178 continue 179 } 180 argRegs = intArgRegs 181 unlock(&finlock) 182 if raceenabled { 183 racefingo() 184 } 185 for fb != nil { 186 for i := fb.cnt; i > 0; i-- { 187 f := &fb.fin[i-1] 188 189 var regs abi.RegArgs 190 var framesz uintptr 191 if argRegs > 0 { 192 // The args can always be passed in registers if they're 193 // available, because platforms we support always have no 194 // argument registers available, or more than 2. 195 // 196 // But unfortunately because we can have an arbitrary 197 // amount of returns and it would be complex to try and 198 // figure out how many of those can get passed in registers, 199 // just conservatively assume none of them do. 200 framesz = f.nret 201 } else { 202 // Need to pass arguments on the stack too. 203 framesz = unsafe.Sizeof((interface{})(nil)) + f.nret 204 } 205 if framecap < framesz { 206 // The frame does not contain pointers interesting for GC, 207 // all not yet finalized objects are stored in finq. 208 // If we do not mark it as FlagNoScan, 209 // the last finalized object is not collected. 210 frame = mallocgc(framesz, nil, true) 211 framecap = framesz 212 } 213 214 if f.fint == nil { 215 throw("missing type in runfinq") 216 } 217 r := frame 218 if argRegs > 0 { 219 r = unsafe.Pointer(®s.Ints) 220 } else { 221 // frame is effectively uninitialized 222 // memory. That means we have to clear 223 // it before writing to it to avoid 224 // confusing the write barrier. 225 *(*[2]uintptr)(frame) = [2]uintptr{} 226 } 227 switch f.fint.kind & kindMask { 228 case kindPtr: 229 // direct use of pointer 230 *(*unsafe.Pointer)(r) = f.arg 231 case kindInterface: 232 ityp := (*interfacetype)(unsafe.Pointer(f.fint)) 233 // set up with empty interface 234 (*eface)(r)._type = &f.ot.typ 235 (*eface)(r).data = f.arg 236 if len(ityp.mhdr) != 0 { 237 // convert to interface with methods 238 // this conversion is guaranteed to succeed - we checked in SetFinalizer 239 (*iface)(r).tab = assertE2I(ityp, (*eface)(r)._type) 240 } 241 default: 242 throw("bad kind in runfinq") 243 } 244 fingRunning = true 245 reflectcall(nil, unsafe.Pointer(f.fn), frame, uint32(framesz), uint32(framesz), uint32(framesz), ®s) 246 fingRunning = false 247 248 // Drop finalizer queue heap references 249 // before hiding them from markroot. 250 // This also ensures these will be 251 // clear if we reuse the finalizer. 252 f.fn = nil 253 f.arg = nil 254 f.ot = nil 255 atomic.Store(&fb.cnt, i-1) 256 } 257 next := fb.next 258 lock(&finlock) 259 fb.next = finc 260 finc = fb 261 unlock(&finlock) 262 fb = next 263 } 264 } 265 } 266 267 // SetFinalizer sets the finalizer associated with obj to the provided 268 // finalizer function. When the garbage collector finds an unreachable block 269 // with an associated finalizer, it clears the association and runs 270 // finalizer(obj) in a separate goroutine. This makes obj reachable again, 271 // but now without an associated finalizer. Assuming that SetFinalizer 272 // is not called again, the next time the garbage collector sees 273 // that obj is unreachable, it will free obj. 274 // 275 // SetFinalizer(obj, nil) clears any finalizer associated with obj. 276 // 277 // The argument obj must be a pointer to an object allocated by calling 278 // new, by taking the address of a composite literal, or by taking the 279 // address of a local variable. 280 // The argument finalizer must be a function that takes a single argument 281 // to which obj's type can be assigned, and can have arbitrary ignored return 282 // values. If either of these is not true, SetFinalizer may abort the 283 // program. 284 // 285 // Finalizers are run in dependency order: if A points at B, both have 286 // finalizers, and they are otherwise unreachable, only the finalizer 287 // for A runs; once A is freed, the finalizer for B can run. 288 // If a cyclic structure includes a block with a finalizer, that 289 // cycle is not guaranteed to be garbage collected and the finalizer 290 // is not guaranteed to run, because there is no ordering that 291 // respects the dependencies. 292 // 293 // The finalizer is scheduled to run at some arbitrary time after the 294 // program can no longer reach the object to which obj points. 295 // There is no guarantee that finalizers will run before a program exits, 296 // so typically they are useful only for releasing non-memory resources 297 // associated with an object during a long-running program. 298 // For example, an os.File object could use a finalizer to close the 299 // associated operating system file descriptor when a program discards 300 // an os.File without calling Close, but it would be a mistake 301 // to depend on a finalizer to flush an in-memory I/O buffer such as a 302 // bufio.Writer, because the buffer would not be flushed at program exit. 303 // 304 // It is not guaranteed that a finalizer will run if the size of *obj is 305 // zero bytes. 306 // 307 // It is not guaranteed that a finalizer will run for objects allocated 308 // in initializers for package-level variables. Such objects may be 309 // linker-allocated, not heap-allocated. 310 // 311 // A finalizer may run as soon as an object becomes unreachable. 312 // In order to use finalizers correctly, the program must ensure that 313 // the object is reachable until it is no longer required. 314 // Objects stored in global variables, or that can be found by tracing 315 // pointers from a global variable, are reachable. For other objects, 316 // pass the object to a call of the KeepAlive function to mark the 317 // last point in the function where the object must be reachable. 318 // 319 // For example, if p points to a struct, such as os.File, that contains 320 // a file descriptor d, and p has a finalizer that closes that file 321 // descriptor, and if the last use of p in a function is a call to 322 // syscall.Write(p.d, buf, size), then p may be unreachable as soon as 323 // the program enters syscall.Write. The finalizer may run at that moment, 324 // closing p.d, causing syscall.Write to fail because it is writing to 325 // a closed file descriptor (or, worse, to an entirely different 326 // file descriptor opened by a different goroutine). To avoid this problem, 327 // call runtime.KeepAlive(p) after the call to syscall.Write. 328 // 329 // A single goroutine runs all finalizers for a program, sequentially. 330 // If a finalizer must run for a long time, it should do so by starting 331 // a new goroutine. 332 func SetFinalizer(obj interface{}, finalizer interface{}) { 333 if debug.sbrk != 0 { 334 // debug.sbrk never frees memory, so no finalizers run 335 // (and we don't have the data structures to record them). 336 return 337 } 338 e := efaceOf(&obj) 339 etyp := e._type 340 if etyp == nil { 341 throw("runtime.SetFinalizer: first argument is nil") 342 } 343 if etyp.kind&kindMask != kindPtr { 344 throw("runtime.SetFinalizer: first argument is " + etyp.string() + ", not pointer") 345 } 346 ot := (*ptrtype)(unsafe.Pointer(etyp)) 347 if ot.elem == nil { 348 throw("nil elem type!") 349 } 350 351 // find the containing object 352 base, _, _ := findObject(uintptr(e.data), 0, 0) 353 354 if base == 0 { 355 // 0-length objects are okay. 356 if e.data == unsafe.Pointer(&zerobase) { 357 return 358 } 359 360 // Global initializers might be linker-allocated. 361 // var Foo = &Object{} 362 // func main() { 363 // runtime.SetFinalizer(Foo, nil) 364 // } 365 // The relevant segments are: noptrdata, data, bss, noptrbss. 366 // We cannot assume they are in any order or even contiguous, 367 // due to external linking. 368 for datap := &firstmoduledata; datap != nil; datap = datap.next { 369 if datap.noptrdata <= uintptr(e.data) && uintptr(e.data) < datap.enoptrdata || 370 datap.data <= uintptr(e.data) && uintptr(e.data) < datap.edata || 371 datap.bss <= uintptr(e.data) && uintptr(e.data) < datap.ebss || 372 datap.noptrbss <= uintptr(e.data) && uintptr(e.data) < datap.enoptrbss { 373 return 374 } 375 } 376 throw("runtime.SetFinalizer: pointer not in allocated block") 377 } 378 379 if uintptr(e.data) != base { 380 // As an implementation detail we allow to set finalizers for an inner byte 381 // of an object if it could come from tiny alloc (see mallocgc for details). 382 if ot.elem == nil || ot.elem.ptrdata != 0 || ot.elem.size >= maxTinySize { 383 throw("runtime.SetFinalizer: pointer not at beginning of allocated block") 384 } 385 } 386 387 f := efaceOf(&finalizer) 388 ftyp := f._type 389 if ftyp == nil { 390 // switch to system stack and remove finalizer 391 systemstack(func() { 392 removefinalizer(e.data) 393 }) 394 return 395 } 396 397 if ftyp.kind&kindMask != kindFunc { 398 throw("runtime.SetFinalizer: second argument is " + ftyp.string() + ", not a function") 399 } 400 ft := (*functype)(unsafe.Pointer(ftyp)) 401 if ft.dotdotdot() { 402 throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string() + " because dotdotdot") 403 } 404 if ft.inCount != 1 { 405 throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string()) 406 } 407 fint := ft.in()[0] 408 switch { 409 case fint == etyp: 410 // ok - same type 411 goto okarg 412 case fint.kind&kindMask == kindPtr: 413 if (fint.uncommon() == nil || etyp.uncommon() == nil) && (*ptrtype)(unsafe.Pointer(fint)).elem == ot.elem { 414 // ok - not same type, but both pointers, 415 // one or the other is unnamed, and same element type, so assignable. 416 goto okarg 417 } 418 case fint.kind&kindMask == kindInterface: 419 ityp := (*interfacetype)(unsafe.Pointer(fint)) 420 if len(ityp.mhdr) == 0 { 421 // ok - satisfies empty interface 422 goto okarg 423 } 424 if iface := assertE2I2(ityp, *efaceOf(&obj)); iface.tab != nil { 425 goto okarg 426 } 427 } 428 throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string()) 429 okarg: 430 // compute size needed for return parameters 431 nret := uintptr(0) 432 for _, t := range ft.out() { 433 nret = alignUp(nret, uintptr(t.align)) + uintptr(t.size) 434 } 435 nret = alignUp(nret, sys.PtrSize) 436 437 // make sure we have a finalizer goroutine 438 createfing() 439 440 systemstack(func() { 441 if !addfinalizer(e.data, (*funcval)(f.data), nret, fint, ot) { 442 throw("runtime.SetFinalizer: finalizer already set") 443 } 444 }) 445 } 446 447 // Mark KeepAlive as noinline so that it is easily detectable as an intrinsic. 448 //go:noinline 449 450 // KeepAlive marks its argument as currently reachable. 451 // This ensures that the object is not freed, and its finalizer is not run, 452 // before the point in the program where KeepAlive is called. 453 // 454 // A very simplified example showing where KeepAlive is required: 455 // type File struct { d int } 456 // d, err := syscall.Open("/file/path", syscall.O_RDONLY, 0) 457 // // ... do something if err != nil ... 458 // p := &File{d} 459 // runtime.SetFinalizer(p, func(p *File) { syscall.Close(p.d) }) 460 // var buf [10]byte 461 // n, err := syscall.Read(p.d, buf[:]) 462 // // Ensure p is not finalized until Read returns. 463 // runtime.KeepAlive(p) 464 // // No more uses of p after this point. 465 // 466 // Without the KeepAlive call, the finalizer could run at the start of 467 // syscall.Read, closing the file descriptor before syscall.Read makes 468 // the actual system call. 469 // 470 // Note: KeepAlive should only be used to prevent finalizers from 471 // running prematurely. In particular, when used with unsafe.Pointer, 472 // the rules for valid uses of unsafe.Pointer still apply. 473 func KeepAlive(x interface{}) { 474 // Introduce a use of x that the compiler can't eliminate. 475 // This makes sure x is alive on entry. We need x to be alive 476 // on entry for "defer runtime.KeepAlive(x)"; see issue 21402. 477 if cgoAlwaysFalse { 478 println(x) 479 } 480 } 481