Initial commit: Go 1.23 release state

src/sync/pool.go

@@ -0,0 +1,318 @@
+// Copyright 2013 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.
+
+package sync
+
+import (
+	"internal/race"
+	"runtime"
+	"sync/atomic"
+	"unsafe"
+)
+
+// A Pool is a set of temporary objects that may be individually saved and
+// retrieved.
+//
+// Any item stored in the Pool may be removed automatically at any time without
+// notification. If the Pool holds the only reference when this happens, the
+// item might be deallocated.
+//
+// A Pool is safe for use by multiple goroutines simultaneously.
+//
+// Pool's purpose is to cache allocated but unused items for later reuse,
+// relieving pressure on the garbage collector. That is, it makes it easy to
+// build efficient, thread-safe free lists. However, it is not suitable for all
+// free lists.
+//
+// An appropriate use of a Pool is to manage a group of temporary items
+// silently shared among and potentially reused by concurrent independent
+// clients of a package. Pool provides a way to amortize allocation overhead
+// across many clients.
+//
+// An example of good use of a Pool is in the fmt package, which maintains a
+// dynamically-sized store of temporary output buffers. The store scales under
+// load (when many goroutines are actively printing) and shrinks when
+// quiescent.
+//
+// On the other hand, a free list maintained as part of a short-lived object is
+// not a suitable use for a Pool, since the overhead does not amortize well in
+// that scenario. It is more efficient to have such objects implement their own
+// free list.
+//
+// A Pool must not be copied after first use.
+//
+// In the terminology of [the Go memory model], a call to Put(x) “synchronizes before”
+// a call to [Pool.Get] returning that same value x.
+// Similarly, a call to New returning x “synchronizes before”
+// a call to Get returning that same value x.
+//
+// [the Go memory model]: https://go.dev/ref/mem
+type Pool struct {
+	noCopy noCopy
+
+	local     unsafe.Pointer // local fixed-size per-P pool, actual type is [P]poolLocal
+	localSize uintptr        // size of the local array
+
+	victim     unsafe.Pointer // local from previous cycle
+	victimSize uintptr        // size of victims array
+
+	// New optionally specifies a function to generate
+	// a value when Get would otherwise return nil.
+	// It may not be changed concurrently with calls to Get.
+	New func() any
+}
+
+// Local per-P Pool appendix.
+type poolLocalInternal struct {
+	private any       // Can be used only by the respective P.
+	shared  poolChain // Local P can pushHead/popHead; any P can popTail.
+}
+
+type poolLocal struct {
+	poolLocalInternal
+
+	// Prevents false sharing on widespread platforms with
+	// 128 mod (cache line size) = 0 .
+	pad [128 - unsafe.Sizeof(poolLocalInternal{})%128]byte
+}
+
+// from runtime
+//
+//go:linkname runtime_randn runtime.randn
+func runtime_randn(n uint32) uint32
+
+var poolRaceHash [128]uint64
+
+// poolRaceAddr returns an address to use as the synchronization point
+// for race detector logic. We don't use the actual pointer stored in x
+// directly, for fear of conflicting with other synchronization on that address.
+// Instead, we hash the pointer to get an index into poolRaceHash.
+// See discussion on golang.org/cl/31589.
+func poolRaceAddr(x any) unsafe.Pointer {
+	ptr := uintptr((*[2]unsafe.Pointer)(unsafe.Pointer(&x))[1])
+	h := uint32((uint64(uint32(ptr)) * 0x85ebca6b) >> 16)
+	return unsafe.Pointer(&poolRaceHash[h%uint32(len(poolRaceHash))])
+}
+
+// Put adds x to the pool.
+func (p *Pool) Put(x any) {
+	if x == nil {
+		return
+	}
+	if race.Enabled {
+		if runtime_randn(4) == 0 {
+			// Randomly drop x on floor.
+			return
+		}
+		race.ReleaseMerge(poolRaceAddr(x))
+		race.Disable()
+	}
+	l, _ := p.pin()
+	if l.private == nil {
+		l.private = x
+	} else {
+		l.shared.pushHead(x)
+	}
+	runtime_procUnpin()
+	if race.Enabled {
+		race.Enable()
+	}
+}
+
+// Get selects an arbitrary item from the [Pool], removes it from the
+// Pool, and returns it to the caller.
+// Get may choose to ignore the pool and treat it as empty.
+// Callers should not assume any relation between values passed to [Pool.Put] and
+// the values returned by Get.
+//
+// If Get would otherwise return nil and p.New is non-nil, Get returns
+// the result of calling p.New.
+func (p *Pool) Get() any {
+	if race.Enabled {
+		race.Disable()
+	}
+	l, pid := p.pin()
+	x := l.private
+	l.private = nil
+	if x == nil {
+		// Try to pop the head of the local shard. We prefer
+		// the head over the tail for temporal locality of
+		// reuse.
+		x, _ = l.shared.popHead()
+		if x == nil {
+			x = p.getSlow(pid)
+		}
+	}
+	runtime_procUnpin()
+	if race.Enabled {
+		race.Enable()
+		if x != nil {
+			race.Acquire(poolRaceAddr(x))
+		}
+	}
+	if x == nil && p.New != nil {
+		x = p.New()
+	}
+	return x
+}
+
+func (p *Pool) getSlow(pid int) any {
+	// See the comment in pin regarding ordering of the loads.
+	size := runtime_LoadAcquintptr(&p.localSize) // load-acquire
+	locals := p.local                            // load-consume
+	// Try to steal one element from other procs.
+	for i := 0; i < int(size); i++ {
+		l := indexLocal(locals, (pid+i+1)%int(size))
+		if x, _ := l.shared.popTail(); x != nil {
+			return x
+		}
+	}
+
+	// Try the victim cache. We do this after attempting to steal
+	// from all primary caches because we want objects in the
+	// victim cache to age out if at all possible.
+	size = atomic.LoadUintptr(&p.victimSize)
+	if uintptr(pid) >= size {
+		return nil
+	}
+	locals = p.victim
+	l := indexLocal(locals, pid)
+	if x := l.private; x != nil {
+		l.private = nil
+		return x
+	}
+	for i := 0; i < int(size); i++ {
+		l := indexLocal(locals, (pid+i)%int(size))
+		if x, _ := l.shared.popTail(); x != nil {
+			return x
+		}
+	}
+
+	// Mark the victim cache as empty for future gets don't bother
+	// with it.
+	atomic.StoreUintptr(&p.victimSize, 0)
+
+	return nil
+}
+
+// pin pins the current goroutine to P, disables preemption and
+// returns poolLocal pool for the P and the P's id.
+// Caller must call runtime_procUnpin() when done with the pool.
+func (p *Pool) pin() (*poolLocal, int) {
+	// Check whether p is nil to get a panic.
+	// Otherwise the nil dereference happens while the m is pinned,
+	// causing a fatal error rather than a panic.
+	if p == nil {
+		panic("nil Pool")
+	}
+
+	pid := runtime_procPin()
+	// In pinSlow we store to local and then to localSize, here we load in opposite order.
+	// Since we've disabled preemption, GC cannot happen in between.
+	// Thus here we must observe local at least as large localSize.
+	// We can observe a newer/larger local, it is fine (we must observe its zero-initialized-ness).
+	s := runtime_LoadAcquintptr(&p.localSize) // load-acquire
+	l := p.local                              // load-consume
+	if uintptr(pid) < s {
+		return indexLocal(l, pid), pid
+	}
+	return p.pinSlow()
+}
+
+func (p *Pool) pinSlow() (*poolLocal, int) {
+	// Retry under the mutex.
+	// Can not lock the mutex while pinned.
+	runtime_procUnpin()
+	allPoolsMu.Lock()
+	defer allPoolsMu.Unlock()
+	pid := runtime_procPin()
+	// poolCleanup won't be called while we are pinned.
+	s := p.localSize
+	l := p.local
+	if uintptr(pid) < s {
+		return indexLocal(l, pid), pid
+	}
+	if p.local == nil {
+		allPools = append(allPools, p)
+	}
+	// If GOMAXPROCS changes between GCs, we re-allocate the array and lose the old one.
+	size := runtime.GOMAXPROCS(0)
+	local := make([]poolLocal, size)
+	atomic.StorePointer(&p.local, unsafe.Pointer(&local[0])) // store-release
+	runtime_StoreReluintptr(&p.localSize, uintptr(size))     // store-release
+	return &local[pid], pid
+}
+
+// poolCleanup should be an internal detail,
+// but widely used packages access it using linkname.
+// Notable members of the hall of shame include:
+//   - github.com/bytedance/gopkg
+//   - github.com/songzhibin97/gkit
+//
+// Do not remove or change the type signature.
+// See go.dev/issue/67401.
+//
+//go:linkname poolCleanup
+func poolCleanup() {
+	// This function is called with the world stopped, at the beginning of a garbage collection.
+	// It must not allocate and probably should not call any runtime functions.
+
+	// Because the world is stopped, no pool user can be in a
+	// pinned section (in effect, this has all Ps pinned).
+
+	// Drop victim caches from all pools.
+	for _, p := range oldPools {
+		p.victim = nil
+		p.victimSize = 0
+	}
+
+	// Move primary cache to victim cache.
+	for _, p := range allPools {
+		p.victim = p.local
+		p.victimSize = p.localSize
+		p.local = nil
+		p.localSize = 0
+	}
+
+	// The pools with non-empty primary caches now have non-empty
+	// victim caches and no pools have primary caches.
+	oldPools, allPools = allPools, nil
+}
+
+var (
+	allPoolsMu Mutex
+
+	// allPools is the set of pools that have non-empty primary
+	// caches. Protected by either 1) allPoolsMu and pinning or 2)
+	// STW.
+	allPools []*Pool
+
+	// oldPools is the set of pools that may have non-empty victim
+	// caches. Protected by STW.
+	oldPools []*Pool
+)
+
+func init() {
+	runtime_registerPoolCleanup(poolCleanup)
+}
+
+func indexLocal(l unsafe.Pointer, i int) *poolLocal {
+	lp := unsafe.Pointer(uintptr(l) + uintptr(i)*unsafe.Sizeof(poolLocal{}))
+	return (*poolLocal)(lp)
+}
+
+// Implemented in runtime.
+func runtime_registerPoolCleanup(cleanup func())
+func runtime_procPin() int
+func runtime_procUnpin()
+
+// The below are implemented in internal/runtime/atomic and the
+// compiler also knows to intrinsify the symbol we linkname into this
+// package.
+
+//go:linkname runtime_LoadAcquintptr internal/runtime/atomic.LoadAcquintptr
+func runtime_LoadAcquintptr(ptr *uintptr) uintptr
+
+//go:linkname runtime_StoreReluintptr internal/runtime/atomic.StoreReluintptr
+func runtime_StoreReluintptr(ptr *uintptr, val uintptr) uintptr