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  	"internal/goarch"
    19  	"internal/goexperiment"
    20  	"unsafe"
    21  )
    22  
    23  // Go uses a hybrid barrier that combines a Yuasa-style deletion
    24  // barrier—which shades the object whose reference is being
    25  // overwritten—with Dijkstra insertion barrier—which shades the object
    26  // whose reference is being written. The insertion part of the barrier
    27  // is necessary while the calling goroutine's stack is grey. In
    28  // pseudocode, the barrier is:
    29  //
    30  //     writePointer(slot, ptr):
    31  //         shade(*slot)
    32  //         if current stack is grey:
    33  //             shade(ptr)
    34  //         *slot = ptr
    35  //
    36  // slot is the destination in Go code.
    37  // ptr is the value that goes into the slot in Go code.
    38  //
    39  // Shade indicates that it has seen a white pointer by adding the referent
    40  // to wbuf as well as marking it.
    41  //
    42  // The two shades and the condition work together to prevent a mutator
    43  // from hiding an object from the garbage collector:
    44  //
    45  // 1. shade(*slot) prevents a mutator from hiding an object by moving
    46  // the sole pointer to it from the heap to its stack. If it attempts
    47  // to unlink an object from the heap, this will shade it.
    48  //
    49  // 2. shade(ptr) prevents a mutator from hiding an object by moving
    50  // the sole pointer to it from its stack into a black object in the
    51  // heap. If it attempts to install the pointer into a black object,
    52  // this will shade it.
    53  //
    54  // 3. Once a goroutine's stack is black, the shade(ptr) becomes
    55  // unnecessary. shade(ptr) prevents hiding an object by moving it from
    56  // the stack to the heap, but this requires first having a pointer
    57  // hidden on the stack. Immediately after a stack is scanned, it only
    58  // points to shaded objects, so it's not hiding anything, and the
    59  // shade(*slot) prevents it from hiding any other pointers on its
    60  // stack.
    61  //
    62  // For a detailed description of this barrier and proof of
    63  // correctness, see https://github.com/golang/proposal/blob/master/design/17503-eliminate-rescan.md
    64  //
    65  //
    66  //
    67  // Dealing with memory ordering:
    68  //
    69  // Both the Yuasa and Dijkstra barriers can be made conditional on the
    70  // color of the object containing the slot. We chose not to make these
    71  // conditional because the cost of ensuring that the object holding
    72  // the slot doesn't concurrently change color without the mutator
    73  // noticing seems prohibitive.
    74  //
    75  // Consider the following example where the mutator writes into
    76  // a slot and then loads the slot's mark bit while the GC thread
    77  // writes to the slot's mark bit and then as part of scanning reads
    78  // the slot.
    79  //
    80  // Initially both [slot] and [slotmark] are 0 (nil)
    81  // Mutator thread          GC thread
    82  // st [slot], ptr          st [slotmark], 1
    83  //
    84  // ld r1, [slotmark]       ld r2, [slot]
    85  //
    86  // Without an expensive memory barrier between the st and the ld, the final
    87  // result on most HW (including 386/amd64) can be r1==r2==0. This is a classic
    88  // example of what can happen when loads are allowed to be reordered with older
    89  // stores (avoiding such reorderings lies at the heart of the classic
    90  // Peterson/Dekker algorithms for mutual exclusion). Rather than require memory
    91  // barriers, which will slow down both the mutator and the GC, we always grey
    92  // the ptr object regardless of the slot's color.
    93  //
    94  // Another place where we intentionally omit memory barriers is when
    95  // accessing mheap_.arena_used to check if a pointer points into the
    96  // heap. On relaxed memory machines, it's possible for a mutator to
    97  // extend the size of the heap by updating arena_used, allocate an
    98  // object from this new region, and publish a pointer to that object,
    99  // but for tracing running on another processor to observe the pointer
   100  // but use the old value of arena_used. In this case, tracing will not
   101  // mark the object, even though it's reachable. However, the mutator
   102  // is guaranteed to execute a write barrier when it publishes the
   103  // pointer, so it will take care of marking the object. A general
   104  // consequence of this is that the garbage collector may cache the
   105  // value of mheap_.arena_used. (See issue #9984.)
   106  //
   107  //
   108  // Stack writes:
   109  //
   110  // The compiler omits write barriers for writes to the current frame,
   111  // but if a stack pointer has been passed down the call stack, the
   112  // compiler will generate a write barrier for writes through that
   113  // pointer (because it doesn't know it's not a heap pointer).
   114  //
   115  //
   116  // Global writes:
   117  //
   118  // The Go garbage collector requires write barriers when heap pointers
   119  // are stored in globals. Many garbage collectors ignore writes to
   120  // globals and instead pick up global -> heap pointers during
   121  // termination. This increases pause time, so we instead rely on write
   122  // barriers for writes to globals so that we don't have to rescan
   123  // global during mark termination.
   124  //
   125  //
   126  // Publication ordering:
   127  //
   128  // The write barrier is *pre-publication*, meaning that the write
   129  // barrier happens prior to the *slot = ptr write that may make ptr
   130  // reachable by some goroutine that currently cannot reach it.
   131  //
   132  //
   133  // Signal handler pointer writes:
   134  //
   135  // In general, the signal handler cannot safely invoke the write
   136  // barrier because it may run without a P or even during the write
   137  // barrier.
   138  //
   139  // There is exactly one exception: profbuf.go omits a barrier during
   140  // signal handler profile logging. That's safe only because of the
   141  // deletion barrier. See profbuf.go for a detailed argument. If we
   142  // remove the deletion barrier, we'll have to work out a new way to
   143  // handle the profile logging.
   144  
   145  // typedmemmove copies a value of type typ to dst from src.
   146  // Must be nosplit, see #16026.
   147  //
   148  // TODO: Perfect for go:nosplitrec since we can't have a safe point
   149  // anywhere in the bulk barrier or memmove.
   150  //
   151  //go:nosplit
   152  func typedmemmove(typ *abi.Type, dst, src unsafe.Pointer) {
   153  	if dst == src {
   154  		return
   155  	}
   156  	if writeBarrier.needed && typ.PtrBytes != 0 {
   157  		bulkBarrierPreWrite(uintptr(dst), uintptr(src), typ.PtrBytes)
   158  	}
   159  	// There's a race here: if some other goroutine can write to
   160  	// src, it may change some pointer in src after we've
   161  	// performed the write barrier but before we perform the
   162  	// memory copy. This safe because the write performed by that
   163  	// other goroutine must also be accompanied by a write
   164  	// barrier, so at worst we've unnecessarily greyed the old
   165  	// pointer that was in src.
   166  	memmove(dst, src, typ.Size_)
   167  	if goexperiment.CgoCheck2 {
   168  		cgoCheckMemmove2(typ, dst, src, 0, typ.Size_)
   169  	}
   170  }
   171  
   172  // wbZero performs the write barrier operations necessary before
   173  // zeroing a region of memory at address dst of type typ.
   174  // Does not actually do the zeroing.
   175  //
   176  //go:nowritebarrierrec
   177  //go:nosplit
   178  func wbZero(typ *_type, dst unsafe.Pointer) {
   179  	bulkBarrierPreWrite(uintptr(dst), 0, typ.PtrBytes)
   180  }
   181  
   182  // wbMove performs the write barrier operations necessary before
   183  // copying a region of memory from src to dst of type typ.
   184  // Does not actually do the copying.
   185  //
   186  //go:nowritebarrierrec
   187  //go:nosplit
   188  func wbMove(typ *_type, dst, src unsafe.Pointer) {
   189  	bulkBarrierPreWrite(uintptr(dst), uintptr(src), typ.PtrBytes)
   190  }
   191  
   192  //go:linkname reflect_typedmemmove reflect.typedmemmove
   193  func reflect_typedmemmove(typ *_type, dst, src unsafe.Pointer) {
   194  	if raceenabled {
   195  		raceWriteObjectPC(typ, dst, getcallerpc(), abi.FuncPCABIInternal(reflect_typedmemmove))
   196  		raceReadObjectPC(typ, src, getcallerpc(), abi.FuncPCABIInternal(reflect_typedmemmove))
   197  	}
   198  	if msanenabled {
   199  		msanwrite(dst, typ.Size_)
   200  		msanread(src, typ.Size_)
   201  	}
   202  	if asanenabled {
   203  		asanwrite(dst, typ.Size_)
   204  		asanread(src, typ.Size_)
   205  	}
   206  	typedmemmove(typ, dst, src)
   207  }
   208  
   209  //go:linkname reflectlite_typedmemmove internal/reflectlite.typedmemmove
   210  func reflectlite_typedmemmove(typ *_type, dst, src unsafe.Pointer) {
   211  	reflect_typedmemmove(typ, dst, src)
   212  }
   213  
   214  // reflectcallmove is invoked by reflectcall to copy the return values
   215  // out of the stack and into the heap, invoking the necessary write
   216  // barriers. dst, src, and size describe the return value area to
   217  // copy. typ describes the entire frame (not just the return values).
   218  // typ may be nil, which indicates write barriers are not needed.
   219  //
   220  // It must be nosplit and must only call nosplit functions because the
   221  // stack map of reflectcall is wrong.
   222  //
   223  //go:nosplit
   224  func reflectcallmove(typ *_type, dst, src unsafe.Pointer, size uintptr, regs *abi.RegArgs) {
   225  	if writeBarrier.needed && typ != nil && typ.PtrBytes != 0 && size >= goarch.PtrSize {
   226  		bulkBarrierPreWrite(uintptr(dst), uintptr(src), size)
   227  	}
   228  	memmove(dst, src, size)
   229  
   230  	// Move pointers returned in registers to a place where the GC can see them.
   231  	for i := range regs.Ints {
   232  		if regs.ReturnIsPtr.Get(i) {
   233  			regs.Ptrs[i] = unsafe.Pointer(regs.Ints[i])
   234  		}
   235  	}
   236  }
   237  
   238  //go:nosplit
   239  func typedslicecopy(typ *_type, dstPtr unsafe.Pointer, dstLen int, srcPtr unsafe.Pointer, srcLen int) int {
   240  	n := dstLen
   241  	if n > srcLen {
   242  		n = srcLen
   243  	}
   244  	if n == 0 {
   245  		return 0
   246  	}
   247  
   248  	// The compiler emits calls to typedslicecopy before
   249  	// instrumentation runs, so unlike the other copying and
   250  	// assignment operations, it's not instrumented in the calling
   251  	// code and needs its own instrumentation.
   252  	if raceenabled {
   253  		callerpc := getcallerpc()
   254  		pc := abi.FuncPCABIInternal(slicecopy)
   255  		racewriterangepc(dstPtr, uintptr(n)*typ.Size_, callerpc, pc)
   256  		racereadrangepc(srcPtr, uintptr(n)*typ.Size_, callerpc, pc)
   257  	}
   258  	if msanenabled {
   259  		msanwrite(dstPtr, uintptr(n)*typ.Size_)
   260  		msanread(srcPtr, uintptr(n)*typ.Size_)
   261  	}
   262  	if asanenabled {
   263  		asanwrite(dstPtr, uintptr(n)*typ.Size_)
   264  		asanread(srcPtr, uintptr(n)*typ.Size_)
   265  	}
   266  
   267  	if goexperiment.CgoCheck2 {
   268  		cgoCheckSliceCopy(typ, dstPtr, srcPtr, n)
   269  	}
   270  
   271  	if dstPtr == srcPtr {
   272  		return n
   273  	}
   274  
   275  	// Note: No point in checking typ.PtrBytes here:
   276  	// compiler only emits calls to typedslicecopy for types with pointers,
   277  	// and growslice and reflect_typedslicecopy check for pointers
   278  	// before calling typedslicecopy.
   279  	size := uintptr(n) * typ.Size_
   280  	if writeBarrier.needed {
   281  		pwsize := size - typ.Size_ + typ.PtrBytes
   282  		bulkBarrierPreWrite(uintptr(dstPtr), uintptr(srcPtr), pwsize)
   283  	}
   284  	// See typedmemmove for a discussion of the race between the
   285  	// barrier and memmove.
   286  	memmove(dstPtr, srcPtr, size)
   287  	return n
   288  }
   289  
   290  //go:linkname reflect_typedslicecopy reflect.typedslicecopy
   291  func reflect_typedslicecopy(elemType *_type, dst, src slice) int {
   292  	if elemType.PtrBytes == 0 {
   293  		return slicecopy(dst.array, dst.len, src.array, src.len, elemType.Size_)
   294  	}
   295  	return typedslicecopy(elemType, dst.array, dst.len, src.array, src.len)
   296  }
   297  
   298  // typedmemclr clears the typed memory at ptr with type typ. The
   299  // memory at ptr must already be initialized (and hence in type-safe
   300  // state). If the memory is being initialized for the first time, see
   301  // memclrNoHeapPointers.
   302  //
   303  // If the caller knows that typ has pointers, it can alternatively
   304  // call memclrHasPointers.
   305  //
   306  // TODO: A "go:nosplitrec" annotation would be perfect for this.
   307  //
   308  //go:nosplit
   309  func typedmemclr(typ *_type, ptr unsafe.Pointer) {
   310  	if writeBarrier.needed && typ.PtrBytes != 0 {
   311  		bulkBarrierPreWrite(uintptr(ptr), 0, typ.PtrBytes)
   312  	}
   313  	memclrNoHeapPointers(ptr, typ.Size_)
   314  }
   315  
   316  //go:linkname reflect_typedmemclr reflect.typedmemclr
   317  func reflect_typedmemclr(typ *_type, ptr unsafe.Pointer) {
   318  	typedmemclr(typ, ptr)
   319  }
   320  
   321  //go:linkname reflect_typedmemclrpartial reflect.typedmemclrpartial
   322  func reflect_typedmemclrpartial(typ *_type, ptr unsafe.Pointer, off, size uintptr) {
   323  	if writeBarrier.needed && typ.PtrBytes != 0 {
   324  		bulkBarrierPreWrite(uintptr(ptr), 0, size)
   325  	}
   326  	memclrNoHeapPointers(ptr, size)
   327  }
   328  
   329  //go:linkname reflect_typedarrayclear reflect.typedarrayclear
   330  func reflect_typedarrayclear(typ *_type, ptr unsafe.Pointer, len int) {
   331  	size := typ.Size_ * uintptr(len)
   332  	if writeBarrier.needed && typ.PtrBytes != 0 {
   333  		bulkBarrierPreWrite(uintptr(ptr), 0, size)
   334  	}
   335  	memclrNoHeapPointers(ptr, size)
   336  }
   337  
   338  // memclrHasPointers clears n bytes of typed memory starting at ptr.
   339  // The caller must ensure that the type of the object at ptr has
   340  // pointers, usually by checking typ.PtrBytes. However, ptr
   341  // does not have to point to the start of the allocation.
   342  //
   343  //go:nosplit
   344  func memclrHasPointers(ptr unsafe.Pointer, n uintptr) {
   345  	bulkBarrierPreWrite(uintptr(ptr), 0, n)
   346  	memclrNoHeapPointers(ptr, n)
   347  }
   348  

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