Source file src/runtime/mbarrier.go

     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  

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