Source file src/runtime/mfinal.go
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 "internal/goarch" 12 "runtime/internal/atomic" 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*goarch.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 / goarch.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*goarch.PtrSize || 99 unsafe.Offsetof(finalizer{}.fn) != 0 || 100 unsafe.Offsetof(finalizer{}.arg) != goarch.PtrSize || 101 unsafe.Offsetof(finalizer{}.nret) != 2*goarch.PtrSize || 102 unsafe.Offsetof(finalizer{}.fint) != 3*goarch.PtrSize || 103 unsafe.Offsetof(finalizer{}.ot) != 4*goarch.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 // The args may be passed in registers or on stack. Even for 191 // the register case, we still need the spill slots. 192 // TODO: revisit if we remove spill slots. 193 // 194 // Unfortunately because we can have an arbitrary 195 // amount of returns and it would be complex to try and 196 // figure out how many of those can get passed in registers, 197 // just conservatively assume none of them do. 198 framesz := unsafe.Sizeof((any)(nil)) + f.nret 199 if framecap < framesz { 200 // The frame does not contain pointers interesting for GC, 201 // all not yet finalized objects are stored in finq. 202 // If we do not mark it as FlagNoScan, 203 // the last finalized object is not collected. 204 frame = mallocgc(framesz, nil, true) 205 framecap = framesz 206 } 207 208 if f.fint == nil { 209 throw("missing type in runfinq") 210 } 211 r := frame 212 if argRegs > 0 { 213 r = unsafe.Pointer(®s.Ints) 214 } else { 215 // frame is effectively uninitialized 216 // memory. That means we have to clear 217 // it before writing to it to avoid 218 // confusing the write barrier. 219 *(*[2]uintptr)(frame) = [2]uintptr{} 220 } 221 switch f.fint.kind & kindMask { 222 case kindPtr: 223 // direct use of pointer 224 *(*unsafe.Pointer)(r) = f.arg 225 case kindInterface: 226 ityp := (*interfacetype)(unsafe.Pointer(f.fint)) 227 // set up with empty interface 228 (*eface)(r)._type = &f.ot.typ 229 (*eface)(r).data = f.arg 230 if len(ityp.mhdr) != 0 { 231 // convert to interface with methods 232 // this conversion is guaranteed to succeed - we checked in SetFinalizer 233 (*iface)(r).tab = assertE2I(ityp, (*eface)(r)._type) 234 } 235 default: 236 throw("bad kind in runfinq") 237 } 238 fingRunning = true 239 reflectcall(nil, unsafe.Pointer(f.fn), frame, uint32(framesz), uint32(framesz), uint32(framesz), ®s) 240 fingRunning = false 241 242 // Drop finalizer queue heap references 243 // before hiding them from markroot. 244 // This also ensures these will be 245 // clear if we reuse the finalizer. 246 f.fn = nil 247 f.arg = nil 248 f.ot = nil 249 atomic.Store(&fb.cnt, i-1) 250 } 251 next := fb.next 252 lock(&finlock) 253 fb.next = finc 254 finc = fb 255 unlock(&finlock) 256 fb = next 257 } 258 } 259 } 260 261 // SetFinalizer sets the finalizer associated with obj to the provided 262 // finalizer function. When the garbage collector finds an unreachable block 263 // with an associated finalizer, it clears the association and runs 264 // finalizer(obj) in a separate goroutine. This makes obj reachable again, 265 // but now without an associated finalizer. Assuming that SetFinalizer 266 // is not called again, the next time the garbage collector sees 267 // that obj is unreachable, it will free obj. 268 // 269 // SetFinalizer(obj, nil) clears any finalizer associated with obj. 270 // 271 // The argument obj must be a pointer to an object allocated by calling 272 // new, by taking the address of a composite literal, or by taking the 273 // address of a local variable. 274 // The argument finalizer must be a function that takes a single argument 275 // to which obj's type can be assigned, and can have arbitrary ignored return 276 // values. If either of these is not true, SetFinalizer may abort the 277 // program. 278 // 279 // Finalizers are run in dependency order: if A points at B, both have 280 // finalizers, and they are otherwise unreachable, only the finalizer 281 // for A runs; once A is freed, the finalizer for B can run. 282 // If a cyclic structure includes a block with a finalizer, that 283 // cycle is not guaranteed to be garbage collected and the finalizer 284 // is not guaranteed to run, because there is no ordering that 285 // respects the dependencies. 286 // 287 // The finalizer is scheduled to run at some arbitrary time after the 288 // program can no longer reach the object to which obj points. 289 // There is no guarantee that finalizers will run before a program exits, 290 // so typically they are useful only for releasing non-memory resources 291 // associated with an object during a long-running program. 292 // For example, an os.File object could use a finalizer to close the 293 // associated operating system file descriptor when a program discards 294 // an os.File without calling Close, but it would be a mistake 295 // to depend on a finalizer to flush an in-memory I/O buffer such as a 296 // bufio.Writer, because the buffer would not be flushed at program exit. 297 // 298 // It is not guaranteed that a finalizer will run if the size of *obj is 299 // zero bytes. 300 // 301 // It is not guaranteed that a finalizer will run for objects allocated 302 // in initializers for package-level variables. Such objects may be 303 // linker-allocated, not heap-allocated. 304 // 305 // A finalizer may run as soon as an object becomes unreachable. 306 // In order to use finalizers correctly, the program must ensure that 307 // the object is reachable until it is no longer required. 308 // Objects stored in global variables, or that can be found by tracing 309 // pointers from a global variable, are reachable. For other objects, 310 // pass the object to a call of the KeepAlive function to mark the 311 // last point in the function where the object must be reachable. 312 // 313 // For example, if p points to a struct, such as os.File, that contains 314 // a file descriptor d, and p has a finalizer that closes that file 315 // descriptor, and if the last use of p in a function is a call to 316 // syscall.Write(p.d, buf, size), then p may be unreachable as soon as 317 // the program enters syscall.Write. The finalizer may run at that moment, 318 // closing p.d, causing syscall.Write to fail because it is writing to 319 // a closed file descriptor (or, worse, to an entirely different 320 // file descriptor opened by a different goroutine). To avoid this problem, 321 // call runtime.KeepAlive(p) after the call to syscall.Write. 322 // 323 // A single goroutine runs all finalizers for a program, sequentially. 324 // If a finalizer must run for a long time, it should do so by starting 325 // a new goroutine. 326 func SetFinalizer(obj any, finalizer any) { 327 if debug.sbrk != 0 { 328 // debug.sbrk never frees memory, so no finalizers run 329 // (and we don't have the data structures to record them). 330 return 331 } 332 e := efaceOf(&obj) 333 etyp := e._type 334 if etyp == nil { 335 throw("runtime.SetFinalizer: first argument is nil") 336 } 337 if etyp.kind&kindMask != kindPtr { 338 throw("runtime.SetFinalizer: first argument is " + etyp.string() + ", not pointer") 339 } 340 ot := (*ptrtype)(unsafe.Pointer(etyp)) 341 if ot.elem == nil { 342 throw("nil elem type!") 343 } 344 345 // find the containing object 346 base, _, _ := findObject(uintptr(e.data), 0, 0) 347 348 if base == 0 { 349 // 0-length objects are okay. 350 if e.data == unsafe.Pointer(&zerobase) { 351 return 352 } 353 354 // Global initializers might be linker-allocated. 355 // var Foo = &Object{} 356 // func main() { 357 // runtime.SetFinalizer(Foo, nil) 358 // } 359 // The relevant segments are: noptrdata, data, bss, noptrbss. 360 // We cannot assume they are in any order or even contiguous, 361 // due to external linking. 362 for datap := &firstmoduledata; datap != nil; datap = datap.next { 363 if datap.noptrdata <= uintptr(e.data) && uintptr(e.data) < datap.enoptrdata || 364 datap.data <= uintptr(e.data) && uintptr(e.data) < datap.edata || 365 datap.bss <= uintptr(e.data) && uintptr(e.data) < datap.ebss || 366 datap.noptrbss <= uintptr(e.data) && uintptr(e.data) < datap.enoptrbss { 367 return 368 } 369 } 370 throw("runtime.SetFinalizer: pointer not in allocated block") 371 } 372 373 if uintptr(e.data) != base { 374 // As an implementation detail we allow to set finalizers for an inner byte 375 // of an object if it could come from tiny alloc (see mallocgc for details). 376 if ot.elem == nil || ot.elem.ptrdata != 0 || ot.elem.size >= maxTinySize { 377 throw("runtime.SetFinalizer: pointer not at beginning of allocated block") 378 } 379 } 380 381 f := efaceOf(&finalizer) 382 ftyp := f._type 383 if ftyp == nil { 384 // switch to system stack and remove finalizer 385 systemstack(func() { 386 removefinalizer(e.data) 387 }) 388 return 389 } 390 391 if ftyp.kind&kindMask != kindFunc { 392 throw("runtime.SetFinalizer: second argument is " + ftyp.string() + ", not a function") 393 } 394 ft := (*functype)(unsafe.Pointer(ftyp)) 395 if ft.dotdotdot() { 396 throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string() + " because dotdotdot") 397 } 398 if ft.inCount != 1 { 399 throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string()) 400 } 401 fint := ft.in()[0] 402 switch { 403 case fint == etyp: 404 // ok - same type 405 goto okarg 406 case fint.kind&kindMask == kindPtr: 407 if (fint.uncommon() == nil || etyp.uncommon() == nil) && (*ptrtype)(unsafe.Pointer(fint)).elem == ot.elem { 408 // ok - not same type, but both pointers, 409 // one or the other is unnamed, and same element type, so assignable. 410 goto okarg 411 } 412 case fint.kind&kindMask == kindInterface: 413 ityp := (*interfacetype)(unsafe.Pointer(fint)) 414 if len(ityp.mhdr) == 0 { 415 // ok - satisfies empty interface 416 goto okarg 417 } 418 if iface := assertE2I2(ityp, *efaceOf(&obj)); iface.tab != nil { 419 goto okarg 420 } 421 } 422 throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string()) 423 okarg: 424 // compute size needed for return parameters 425 nret := uintptr(0) 426 for _, t := range ft.out() { 427 nret = alignUp(nret, uintptr(t.align)) + uintptr(t.size) 428 } 429 nret = alignUp(nret, goarch.PtrSize) 430 431 // make sure we have a finalizer goroutine 432 createfing() 433 434 systemstack(func() { 435 if !addfinalizer(e.data, (*funcval)(f.data), nret, fint, ot) { 436 throw("runtime.SetFinalizer: finalizer already set") 437 } 438 }) 439 } 440 441 // Mark KeepAlive as noinline so that it is easily detectable as an intrinsic. 442 //go:noinline 443 444 // KeepAlive marks its argument as currently reachable. 445 // This ensures that the object is not freed, and its finalizer is not run, 446 // before the point in the program where KeepAlive is called. 447 // 448 // A very simplified example showing where KeepAlive is required: 449 // type File struct { d int } 450 // d, err := syscall.Open("/file/path", syscall.O_RDONLY, 0) 451 // // ... do something if err != nil ... 452 // p := &File{d} 453 // runtime.SetFinalizer(p, func(p *File) { syscall.Close(p.d) }) 454 // var buf [10]byte 455 // n, err := syscall.Read(p.d, buf[:]) 456 // // Ensure p is not finalized until Read returns. 457 // runtime.KeepAlive(p) 458 // // No more uses of p after this point. 459 // 460 // Without the KeepAlive call, the finalizer could run at the start of 461 // syscall.Read, closing the file descriptor before syscall.Read makes 462 // the actual system call. 463 // 464 // Note: KeepAlive should only be used to prevent finalizers from 465 // running prematurely. In particular, when used with unsafe.Pointer, 466 // the rules for valid uses of unsafe.Pointer still apply. 467 func KeepAlive(x any) { 468 // Introduce a use of x that the compiler can't eliminate. 469 // This makes sure x is alive on entry. We need x to be alive 470 // on entry for "defer runtime.KeepAlive(x)"; see issue 21402. 471 if cgoAlwaysFalse { 472 println(x) 473 } 474 } 475