runtime: don't hold global gcBitsArenas lock over allocation

Currently, newArena holds the gcBitsArenas lock across allocating
memory from the OS for a new gcBits arena. This is a global lock and
allocating physical memory can be expensive, so this has the potential
to cause high lock contention, especially since every single span
sweep operation calls newArena (via newMarkBits).

Improve the situation by temporarily dropping the lock across
allocation. This means the caller now has to revalidate its
assumptions after the lock is dropped, so this also factors out that
code path and reinvokes it after the lock is acquired.

Change-Id: I1113200a954ab4aad16b5071512583cfac744bdc
Reviewed-on: https://go-review.googlesource.com/34594
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mheap.go

@@ -1331,7 +1331,7 @@ type gcBitsHeader struct {
 //go:notinheap
 type gcBits struct {
 	// gcBitsHeader // side step recursive type bug (issue 14620) by including fields by hand.
-	free uintptr // free is the index into bits of the next free byte.
+	free uintptr // free is the index into bits of the next free byte; protected by gcBitsArenas.lock
 	next *gcBits
 	bits [gcBitsChunkBytes - gcBitsHeaderBytes]uint8
 }
@@ -1344,27 +1344,54 @@ var gcBitsArenas struct {
 	previous *gcBits
 }
 
+// tryAlloc allocates from b or returns nil if b does not have enough room.
+// The caller must hold gcBitsArenas.lock.
+func (b *gcBits) tryAlloc(bytes uintptr) *uint8 {
+	if b == nil || b.free+bytes > uintptr(len(b.bits)) {
+		return nil
+	}
+	p := &b.bits[b.free]
+	b.free += bytes
+	return p
+}
+
 // newMarkBits returns a pointer to 8 byte aligned bytes
 // to be used for a span's mark bits.
 func newMarkBits(nelems uintptr) *uint8 {
 	lock(&gcBitsArenas.lock)
 	blocksNeeded := uintptr((nelems + 63) / 64)
 	bytesNeeded := blocksNeeded * 8
-	if gcBitsArenas.next == nil ||
-		gcBitsArenas.next.free+bytesNeeded > uintptr(len(gcBits{}.bits)) {
-		// Allocate a new arena.
-		fresh := newArena()
-		fresh.next = gcBitsArenas.next
-		gcBitsArenas.next = fresh
+	if p := gcBitsArenas.next.tryAlloc(bytesNeeded); p != nil {
+		unlock(&gcBitsArenas.lock)
+		return p
 	}
-	if gcBitsArenas.next.free >= gcBitsChunkBytes {
-		println("runtime: gcBitsArenas.next.free=", gcBitsArenas.next.free, gcBitsChunkBytes)
+
+	// Allocate a new arena. This may temporarily drop the lock.
+	fresh := newArenaMayUnlock()
+	// If newArenaMayUnlock dropped the lock, another thread may
+	// have put a fresh arena on the "next" list. Try allocating
+	// from next again.
+	if p := gcBitsArenas.next.tryAlloc(bytesNeeded); p != nil {
+		// Put fresh back on the free list.
+		// TODO: Mark it "already zeroed"
+		fresh.next = gcBitsArenas.free
+		gcBitsArenas.free = fresh
+		unlock(&gcBitsArenas.lock)
+		return p
+	}
+
+	// Allocate from the fresh arena.
+	p := fresh.tryAlloc(bytesNeeded)
+	if p == nil {
 		throw("markBits overflow")
 	}
-	result := &gcBitsArenas.next.bits[gcBitsArenas.next.free]
-	gcBitsArenas.next.free += bytesNeeded
+
+	// Add the fresh arena to the "next" list.
+	fresh.next = gcBitsArenas.next
+	gcBitsArenas.next = fresh
+
 	unlock(&gcBitsArenas.lock)
-	return result
+	return p
 }
 
 // newAllocBits returns a pointer to 8 byte aligned bytes
@@ -1411,14 +1438,17 @@ func nextMarkBitArenaEpoch() {
 	unlock(&gcBitsArenas.lock)
 }
 
-// newArena allocates and zeroes a gcBits arena.
-func newArena() *gcBits {
+// newArenaMayUnlock allocates and zeroes a gcBits arena.
+// The caller must hold gcBitsArena.lock. This may temporarily release it.
+func newArenaMayUnlock() *gcBits {
 	var result *gcBits
 	if gcBitsArenas.free == nil {
+		unlock(&gcBitsArenas.lock)
 		result = (*gcBits)(sysAlloc(gcBitsChunkBytes, &memstats.gc_sys))
 		if result == nil {
 			throw("runtime: cannot allocate memory")
 		}
+		lock(&gcBitsArenas.lock)
 	} else {
 		result = gcBitsArenas.free
 		gcBitsArenas.free = gcBitsArenas.free.next