multi Nios connect to One Pcie

Single word read/writes are atomic.

The Avalon switch fabric will arbitrate the cycles for you.

Remember it is a slave arbited bus - ie there is arbitration for every slave, not every master. This means that different masters can access different slaves concurrently.

If you have a lot of slaves that aren't used much, then put them behind a bridge to you only have one large slave arbiter (for the bridge), not one on each slave.

For synchonisation you probably need to use Dekker's algorithm since the the avalon bus doesn't support RMW cycles. Although you could add an Avalon slave that implemented mutexes.

Read: return value, set to 1

Write: set to 0

should do it??