src/runtime/mbarrier.go - go - Git at Google

 // 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)
 }
	// 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)
	}