Adding upstream version 1.16.10.upstream/1.16.10upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 327 insertions, 0 deletions

diff --git a/src/runtime/mbarrier.go b/src/runtime/mbarrier.go
new file mode 100644
index 0000000..2b5affc
--- /dev/null
+++ b/src/runtime/mbarrier.go
@@ -0,0 +1,327 @@
+// 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 writeBarrier.needed && typ.ptrdata != 0 {
+		bulkBarrierPreWrite(uintptr(dst), uintptr(src), typ.ptrdata)
+	}
+	// 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.
+// off must be a multiple of sys.PtrSize.
+//go:linkname reflect_typedmemmovepartial reflect.typedmemmovepartial
+func reflect_typedmemmovepartial(typ *_type, dst, src unsafe.Pointer, off, size uintptr) {
+	if writeBarrier.needed && typ.ptrdata > off && size >= sys.PtrSize {
+		if off&(sys.PtrSize-1) != 0 {
+			panic("reflect: internal error: misaligned offset")
+		}
+		pwsize := alignDown(size, sys.PtrSize)
+		if poff := typ.ptrdata - off; pwsize > poff {
+			pwsize = poff
+		}
+		bulkBarrierPreWrite(uintptr(dst), uintptr(src), pwsize)
+	}
+
+	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, dstPtr unsafe.Pointer, dstLen int, srcPtr unsafe.Pointer, srcLen int) int {
+	n := dstLen
+	if n > srcLen {
+		n = srcLen
+	}
+	if n == 0 {
+		return 0
+	}
+
+	// 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(dstPtr, uintptr(n)*typ.size, callerpc, pc)
+		racereadrangepc(srcPtr, uintptr(n)*typ.size, callerpc, pc)
+	}
+	if msanenabled {
+		msanwrite(dstPtr, uintptr(n)*typ.size)
+		msanread(srcPtr, uintptr(n)*typ.size)
+	}
+
+	if writeBarrier.cgo {
+		cgoCheckSliceCopy(typ, dstPtr, srcPtr, n)
+	}
+
+	if dstPtr == srcPtr {
+		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 {
+		pwsize := size - typ.size + typ.ptrdata
+		bulkBarrierPreWrite(uintptr(dstPtr), uintptr(srcPtr), pwsize)
+	}
+	// See typedmemmove for a discussion of the race between the
+	// barrier and memmove.
+	memmove(dstPtr, srcPtr, size)
+	return n
+}
+
+//go:linkname reflect_typedslicecopy reflect.typedslicecopy
+func reflect_typedslicecopy(elemType *_type, dst, src slice) int {
+	if elemType.ptrdata == 0 {
+		return slicecopy(dst.array, dst.len, src.array, src.len, elemType.size)
+	}
+	return typedslicecopy(elemType, dst.array, dst.len, src.array, src.len)
+}
+
+// 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 writeBarrier.needed && typ.ptrdata != 0 {
+		bulkBarrierPreWrite(uintptr(ptr), 0, typ.ptrdata)
+	}
+	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 writeBarrier.needed && 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)
+}