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// 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. This file contains the actual write barrier
// implementation, markwb, and the various wrappers called by the
// compiler to implement pointer assignment, slice assignment,
// typed memmove, and so on.
package runtime
import (
"runtime/internal/sys"
"unsafe"
)
// markwb is the mark-phase write barrier, the only barrier we have.
// The rest of this file exists only to make calls to this function.
//
// This is the Dijkstra barrier coarsened to always shade the ptr (dst) object.
// The original Dijkstra barrier only shaded ptrs being placed in black slots.
//
// Shade indicates that it has seen a white pointer by adding the referent
// to wbuf as well as marking it.
//
// slot is the destination (dst) in go code
// ptr is the value that goes into the slot (src) in the go code
//
//
// Dealing with memory ordering:
//
// Dijkstra pointed out that maintaining the no black to white
// pointers means that white to white pointers do not need
// to be noted by the write barrier. Furthermore if either
// white object dies before it is reached by the
// GC then the object can be collected during this GC cycle
// instead of waiting for the next cycle. Unfortunately the cost of
// ensuring that the object holding the slot doesn't concurrently
// change to black 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.
//
//go:nowritebarrierrec
func gcmarkwb_m(slot *uintptr, ptr uintptr) {
if writeBarrier.needed {
if ptr != 0 && inheap(ptr) {
shade(ptr)
}
}
}
// Write barrier calls must not happen during critical GC and scheduler
// related operations. In particular there are times when the GC assumes
// that the world is stopped but scheduler related code is still being
// executed, dealing with syscalls, dealing with putting gs on runnable
// queues and so forth. This code cannot execute write barriers because
// the GC might drop them on the floor. Stopping the world involves removing
// the p associated with an m. We use the fact that m.p == nil to indicate
// that we are in one these critical section and throw if the write is of
// a pointer to a heap object.
//go:nosplit
func writebarrierptr_nostore1(dst *uintptr, src uintptr) {
mp := acquirem()
if mp.inwb || mp.dying > 0 {
releasem(mp)
return
}
systemstack(func() {
if mp.p == 0 && memstats.enablegc && !mp.inwb && inheap(src) {
throw("writebarrierptr_nostore1 called with mp.p == nil")
}
mp.inwb = true
gcmarkwb_m(dst, src)
})
mp.inwb = false
releasem(mp)
}
// NOTE: Really dst *unsafe.Pointer, src unsafe.Pointer,
// but if we do that, Go inserts a write barrier on *dst = src.
//go:nosplit
func writebarrierptr(dst *uintptr, src uintptr) {
*dst = src
if writeBarrier.cgo {
cgoCheckWriteBarrier(dst, src)
}
if !writeBarrier.needed {
return
}
if src != 0 && src < minPhysPageSize {
systemstack(func() {
print("runtime: writebarrierptr *", dst, " = ", hex(src), "\n")
throw("bad pointer in write barrier")
})
}
writebarrierptr_nostore1(dst, src)
}
// Like writebarrierptr, but the store has already been applied.
// Do not reapply.
//go:nosplit
func writebarrierptr_nostore(dst *uintptr, src uintptr) {
if writeBarrier.cgo {
cgoCheckWriteBarrier(dst, src)
}
if !writeBarrier.needed {
return
}
if src != 0 && src < minPhysPageSize {
systemstack(func() { throw("bad pointer in write barrier") })
}
writebarrierptr_nostore1(dst, src)
}
// typedmemmove copies a value of type t to dst from src.
//go:nosplit
func typedmemmove(typ *_type, dst, src unsafe.Pointer) {
memmove(dst, src, typ.size)
if writeBarrier.cgo {
cgoCheckMemmove(typ, dst, src, 0, typ.size)
}
if typ.kind&kindNoPointers != 0 {
return
}
heapBitsBulkBarrier(uintptr(dst), typ.size)
}
//go:linkname reflect_typedmemmove reflect.typedmemmove
func reflect_typedmemmove(typ *_type, dst, src unsafe.Pointer) {
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) {
memmove(dst, src, size)
if writeBarrier.cgo {
cgoCheckMemmove(typ, dst, src, off, size)
}
if !writeBarrier.needed || typ.kind&kindNoPointers != 0 || size < sys.PtrSize {
return
}
if frag := -off & (sys.PtrSize - 1); frag != 0 {
dst = add(dst, frag)
size -= frag
}
heapBitsBulkBarrier(uintptr(dst), size&^(sys.PtrSize-1))
}
// callwritebarrier is invoked at the end of reflectcall, to execute
// write barrier operations to record the fact that a call's return
// values have just been copied to frame, starting at retoffset
// and continuing to framesize. The entire frame (not just the return
// values) is described by typ. Because the copy has already
// happened, we call writebarrierptr_nostore, and this is nosplit so
// the copy and write barrier appear atomic to GC.
//go:nosplit
func callwritebarrier(typ *_type, frame unsafe.Pointer, framesize, retoffset uintptr) {
if !writeBarrier.needed || typ == nil || typ.kind&kindNoPointers != 0 || framesize-retoffset < sys.PtrSize {
return
}
heapBitsBulkBarrier(uintptr(add(frame, retoffset)), framesize-retoffset)
}
//go:nosplit
func typedslicecopy(typ *_type, dst, src slice) int {
// TODO(rsc): If typedslicecopy becomes faster than calling
// typedmemmove repeatedly, consider using during func growslice.
n := dst.len
if n > src.len {
n = src.len
}
if n == 0 {
return 0
}
dstp := dst.array
srcp := src.array
if raceenabled {
callerpc := getcallerpc(unsafe.Pointer(&typ))
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)
}
// Note: No point in checking typ.kind&kindNoPointers here:
// compiler only emits calls to typedslicecopy for types with pointers,
// and growslice and reflect_typedslicecopy check for pointers
// before calling typedslicecopy.
if !writeBarrier.needed {
memmove(dstp, srcp, uintptr(n)*typ.size)
return n
}
systemstack(func() {
if uintptr(srcp) < uintptr(dstp) && uintptr(srcp)+uintptr(n)*typ.size > uintptr(dstp) {
// Overlap with src before dst.
// Copy backward, being careful not to move dstp/srcp
// out of the array they point into.
dstp = add(dstp, uintptr(n-1)*typ.size)
srcp = add(srcp, uintptr(n-1)*typ.size)
i := 0
for {
typedmemmove(typ, dstp, srcp)
if i++; i >= n {
break
}
dstp = add(dstp, -typ.size)
srcp = add(srcp, -typ.size)
}
} else {
// Copy forward, being careful not to move dstp/srcp
// out of the array they point into.
i := 0
for {
typedmemmove(typ, dstp, srcp)
if i++; i >= n {
break
}
dstp = add(dstp, typ.size)
srcp = add(srcp, typ.size)
}
}
})
return n
}
//go:linkname reflect_typedslicecopy reflect.typedslicecopy
func reflect_typedslicecopy(elemType *_type, dst, src slice) int {
if elemType.kind&kindNoPointers != 0 {
n := dst.len
if n > src.len {
n = src.len
}
memmove(dst.array, src.array, uintptr(n)*elemType.size)
return n
}
return typedslicecopy(elemType, dst, src)
}