// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Garbage collector: write barriers.
//
// For the concurrent garbage collector, the Go compiler implements
// updates to pointer-valued fields that may be in heap objects by
// emitting calls to write barriers. The main write barrier for
// individual pointer writes is gcWriteBarrier and is implemented in
// assembly. This file contains write barrier entry points for bulk
// operations. See also mwbbuf.go.
package runtime
import (
"runtime/internal/sys"
"unsafe"
)
// Go uses a hybrid barrier that combines a Yuasa-style deletion
// barrier—which shades the object whose reference is being
// overwritten—with Dijkstra insertion barrier—which shades the object
// whose reference is being written. The insertion part of the barrier
// is necessary while the calling goroutine's stack is grey. In
// pseudocode, the barrier is:
//
// writePointer(slot, ptr):
// shade(*slot)
// if current stack is grey:
// shade(ptr)
// *slot = ptr
//
// slot is the destination in Go code.
// ptr is the value that goes into the slot in Go code.
//
// Shade indicates that it has seen a white pointer by adding the referent
// to wbuf as well as marking it.
//
// The two shades and the condition work together to prevent a mutator
// from hiding an object from the garbage collector:
//
// 1. shade(*slot) prevents a mutator from hiding an object by moving
// the sole pointer to it from the heap to its stack. If it attempts
// to unlink an object from the heap, this will shade it.
//
// 2. shade(ptr) prevents a mutator from hiding an object by moving
// the sole pointer to it from its stack into a black object in the
// heap. If it attempts to install the pointer into a black object,
// this will shade it.
//
// 3. Once a goroutine's stack is black, the shade(ptr) becomes
// unnecessary. shade(ptr) prevents hiding an object by moving it from
// the stack to the heap, but this requires first having a pointer
// hidden on the stack. Immediately after a stack is scanned, it only
// points to shaded objects, so it's not hiding anything, and the
// shade(*slot) prevents it from hiding any other pointers on its
// stack.
//
// For a detailed description of this barrier and proof of
// correctness, see https://github.com/golang/proposal/blob/master/design/17503-eliminate-rescan.md
//
//
//
// Dealing with memory ordering:
//
// Both the Yuasa and Dijkstra barriers can be made conditional on the
// color of the object containing the slot. We chose not to make these
// conditional because the cost of ensuring that the object holding
// the slot doesn't concurrently change color without the mutator
// noticing seems prohibitive.
//
// Consider the following example where the mutator writes into
// a slot and then loads the slot's mark bit while the GC thread
// writes to the slot's mark bit and then as part of scanning reads
// the slot.
//
// Initially both [slot] and [slotmark] are 0 (nil)
// Mutator thread GC thread
// st [slot], ptr st [slotmark], 1
//
// ld r1, [slotmark] ld r2, [slot]
//
// Without an expensive memory barrier between the st and the ld, the final
// result on most HW (including 386/amd64) can be r1==r2==0. This is a classic
// example of what can happen when loads are allowed to be reordered with older
// stores (avoiding such reorderings lies at the heart of the classic
// Peterson/Dekker algorithms for mutual exclusion). Rather than require memory
// barriers, which will slow down both the mutator and the GC, we always grey
// the ptr object regardless of the slot's color.
//
// Another place where we intentionally omit memory barriers is when
// accessing mheap_.arena_used to check if a pointer points into the
// heap. On relaxed memory machines, it's possible for a mutator to
// extend the size of the heap by updating arena_used, allocate an
// object from this new region, and publish a pointer to that object,
// but for tracing running on another processor to observe the pointer
// but use the old value of arena_used. In this case, tracing will not
// mark the object, even though it's reachable. However, the mutator
// is guaranteed to execute a write barrier when it publishes the
// pointer, so it will take care of marking the object. A general
// consequence of this is that the garbage collector may cache the
// value of mheap_.arena_used. (See issue #9984.)
//
//
// Stack writes:
//
// The compiler omits write barriers for writes to the current frame,
// but if a stack pointer has been passed down the call stack, the
// compiler will generate a write barrier for writes through that
// pointer (because it doesn't know it's not a heap pointer).
//
// One might be tempted to ignore the write barrier if slot points
// into to the stack. Don't do it! Mark termination only re-scans
// frames that have potentially been active since the concurrent scan,
// so it depends on write barriers to track changes to pointers in
// stack frames that have not been active.
//
//
// Global writes:
//
// The Go garbage collector requires write barriers when heap pointers
// are stored in globals. Many garbage collectors ignore writes to
// globals and instead pick up global -> heap pointers during
// termination. This increases pause time, so we instead rely on write
// barriers for writes to globals so that we don't have to rescan
// global during mark termination.
//
//
// Publication ordering:
//
// The write barrier is *pre-publication*, meaning that the write
// barrier happens prior to the *slot = ptr write that may make ptr
// reachable by some goroutine that currently cannot reach it.
//
//
// Signal handler pointer writes:
//
// In general, the signal handler cannot safely invoke the write
// barrier because it may run without a P or even during the write
// barrier.
//
// There is exactly one exception: profbuf.go omits a barrier during
// signal handler profile logging. That's safe only because of the
// deletion barrier. See profbuf.go for a detailed argument. If we
// remove the deletion barrier, we'll have to work out a new way to
// handle the profile logging.
// typedmemmove copies a value of type t to dst from src.
// Must be nosplit, see #16026.
//
// TODO: Perfect for go:nosplitrec since we can't have a safe point
// anywhere in the bulk barrier or memmove.
//
//go:nosplit
func typedmemmove(typ *_type, dst, src unsafe.Pointer) {
if dst == src {
return
}
if typ.ptrdata != 0 {
bulkBarrierPreWrite(uintptr(dst), uintptr(src), typ.size)
}
// There's a race here: if some other goroutine can write to
// src, it may change some pointer in src after we've
// performed the write barrier but before we perform the
// memory copy. This safe because the write performed by that
// other goroutine must also be accompanied by a write
// barrier, so at worst we've unnecessarily greyed the old
// pointer that was in src.
memmove(dst, src, typ.size)
if writeBarrier.cgo {
cgoCheckMemmove(typ, dst, src, 0, typ.size)
}
}
//go:linkname reflect_typedmemmove reflect.typedmemmove
func reflect_typedmemmove(typ *_type, dst, src unsafe.Pointer) {
if raceenabled {
raceWriteObjectPC(typ, dst, getcallerpc(), funcPC(reflect_typedmemmove))
raceReadObjectPC(typ, src, getcallerpc(), funcPC(reflect_typedmemmove))
}
if msanenabled {
msanwrite(dst, typ.size)
msanread(src, typ.size)
}
typedmemmove(typ, dst, src)
}
//go:linkname reflectlite_typedmemmove internal/reflectlite.typedmemmove
func reflectlite_typedmemmove(typ *_type, dst, src unsafe.Pointer) {
reflect_typedmemmove(typ, dst, src)
}
// typedmemmovepartial is like typedmemmove but assumes that
// dst and src point off bytes into the value and only copies size bytes.
//go:linkname reflect_typedmemmovepartial reflect.typedmemmovepartial
func reflect_typedmemmovepartial(typ *_type, dst, src unsafe.Pointer, off, size uintptr) {
if writeBarrier.needed && typ.ptrdata != 0 && size >= sys.PtrSize {
// Pointer-align start address for bulk barrier.
adst, asrc, asize := dst, src, size
if frag := -off & (sys.PtrSize - 1); frag != 0 {
adst = add(dst, frag)
asrc = add(src, frag)
asize -= frag
}
bulkBarrierPreWrite(uintptr(adst), uintptr(asrc), asize&^(sys.PtrSize-1))
}
memmove(dst, src, size)
if writeBarrier.cgo {
cgoCheckMemmove(typ, dst, src, off, size)
}
}
// reflectcallmove is invoked by reflectcall to copy the return values
// out of the stack and into the heap, invoking the necessary write
// barriers. dst, src, and size describe the return value area to
// copy. typ describes the entire frame (not just the return values).
// typ may be nil, which indicates write barriers are not needed.
//
// It must be nosplit and must only call nosplit functions because the
// stack map of reflectcall is wrong.
//
//go:nosplit
func reflectcallmove(typ *_type, dst, src unsafe.Pointer, size uintptr) {
if writeBarrier.needed && typ != nil && typ.ptrdata != 0 && size >= sys.PtrSize {
bulkBarrierPreWrite(uintptr(dst), uintptr(src), size)
}
memmove(dst, src, size)
}
//go:nosplit
func typedslicecopy(typ *_type, dst, src slice) int {
n := dst.len
if n > src.len {
n = src.len
}
if n == 0 {
return 0
}
dstp := dst.array
srcp := src.array
// The compiler emits calls to typedslicecopy before
// instrumentation runs, so unlike the other copying and
// assignment operations, it's not instrumented in the calling
// code and needs its own instrumentation.
if raceenabled {
callerpc := getcallerpc()
pc := funcPC(slicecopy)
racewriterangepc(dstp, uintptr(n)*typ.size, callerpc, pc)
racereadrangepc(srcp, uintptr(n)*typ.size, callerpc, pc)
}
if msanenabled {
msanwrite(dstp, uintptr(n)*typ.size)
msanread(srcp, uintptr(n)*typ.size)
}
if writeBarrier.cgo {
cgoCheckSliceCopy(typ, dst, src, n)
}
if dstp == srcp {
return n
}
// Note: No point in checking typ.ptrdata here:
// compiler only emits calls to typedslicecopy for types with pointers,
// and growslice and reflect_typedslicecopy check for pointers
// before calling typedslicecopy.
size := uintptr(n) * typ.size
if writeBarrier.needed {
bulkBarrierPreWrite(uintptr(dstp), uintptr(srcp), size)
}
// See typedmemmove for a discussion of the race between the
// barrier and memmove.
memmove(dstp, srcp, size)
return n
}
//go:linkname reflect_typedslicecopy reflect.typedslicecopy
func reflect_typedslicecopy(elemType *_type, dst, src slice) int {
if elemType.ptrdata == 0 {
n := dst.len
if n > src.len {
n = src.len
}
if n == 0 {
return 0
}
size := uintptr(n) * elemType.size
if raceenabled {
callerpc := getcallerpc()
pc := funcPC(reflect_typedslicecopy)
racewriterangepc(dst.array, size, callerpc, pc)
racereadrangepc(src.array, size, callerpc, pc)
}
if msanenabled {
msanwrite(dst.array, size)
msanread(src.array, size)
}
memmove(dst.array, src.array, size)
return n
}
return typedslicecopy(elemType, dst, src)
}
// typedmemclr clears the typed memory at ptr with type typ. The
// memory at ptr must already be initialized (and hence in type-safe
// state). If the memory is being initialized for the first time, see
// memclrNoHeapPointers.
//
// If the caller knows that typ has pointers, it can alternatively
// call memclrHasPointers.
//
//go:nosplit
func typedmemclr(typ *_type, ptr unsafe.Pointer) {
if typ.ptrdata != 0 {
bulkBarrierPreWrite(uintptr(ptr), 0, typ.size)
}
memclrNoHeapPointers(ptr, typ.size)
}
//go:linkname reflect_typedmemclr reflect.typedmemclr
func reflect_typedmemclr(typ *_type, ptr unsafe.Pointer) {
typedmemclr(typ, ptr)
}
//go:linkname reflect_typedmemclrpartial reflect.typedmemclrpartial
func reflect_typedmemclrpartial(typ *_type, ptr unsafe.Pointer, off, size uintptr) {
if typ.ptrdata != 0 {
bulkBarrierPreWrite(uintptr(ptr), 0, size)
}
memclrNoHeapPointers(ptr, size)
}
// memclrHasPointers clears n bytes of typed memory starting at ptr.
// The caller must ensure that the type of the object at ptr has
// pointers, usually by checking typ.ptrdata. However, ptr
// does not have to point to the start of the allocation.
//
//go:nosplit
func memclrHasPointers(ptr unsafe.Pointer, n uintptr) {
bulkBarrierPreWrite(uintptr(ptr), 0, n)
memclrNoHeapPointers(ptr, n)
}
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