Function storeSeqlockSection

fbclock/shmem_notlinux.go:165–184

storeSeqlockSection writes a data section guarded by the seqlock at seqOff: bump the counter odd, run write (which gets the section's data slice starting right after the 8-byte counter), bump it even.

(data []byte, seqOff int, write func(buf []byte))

// bump the counter odd, run write (which gets the section's data slice starting
// right after the 8-byte counter), bump it even.
func storeSeqlockSection(data []byte, seqOff int, write func(buf []byte)) error {
	seqPtr := (*uint64)(unsafe.Pointer(&data[seqOff]))
	for range maxReadTries {
		seq := atomic.LoadUint64(seqPtr)
		if seq&1 != 0 {
			time.Sleep(time.Microsecond)
			continue
		}
		seq = (seq &^ 1) + 1
		atomic.StoreUint64(seqPtr, seq)
		seq++
		write(data[seqOff+8:])
		if seq == 0 {
			seq = 2
		}
		atomic.StoreUint64(seqPtr, seq)
		return nil
	}
	return fmt.Errorf("seqlock contention after %d tries", maxReadTries)
}

// loadSeqlockSection returns a fresh copy of dataLen bytes from the section
// guarded by the seqlock at seqOff, retrying if a write raced the read.