# Deadlock (MPI)
The classic multithreaded deadlock: thread A holds mutex 1 and waits for mutex 2, while thread B holds mutex 2 and waits for mutex 1. Neither can proceed because each is blocked on a resource the other holds. MPI deadlocks follow the same pattern with blocking sends taking the place of mutex acquires. If process 0 calls `MPI_Send` to process 1 while process 1 simultaneously calls `MPI_Send` to process 0, both block waiting for the peer to call `MPI_Recv`, and neither ever does.

```c
// deadlock: both processes block on MPI_Send waiting for the other to MPI_Recv
MPI_Send(buf, N, MPI_DOUBLE, peer, 0, MPI_COMM_WORLD);
MPI_Recv(buf, N, MPI_DOUBLE, peer, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
```

The standard fixes are: use `MPI_Sendrecv`, which handles ordering internally; post a non-blocking `MPI_Isend` first, then `MPI_Recv`, then `MPI_Wait`; or alternate send/receive order by rank so even-rank processes send first and odd-rank processes receive first. Whether `MPI_Send` actually blocks depends on the implementation's internal buffer size, so relying on it to absorb a large message is non-portable and can silently deadlock on a different machine.