死锁——1960年代，为什么两个程序都没有出错，整个计算机却停止了工作？

lock_x.acquire()
lock_y.acquire()

# 使用 X 和 Y

lock_y.release()
lock_x.release()

lock_y.acquire()
lock_x.acquire()

# 使用 Y 和 X

lock_x.release()
lock_y.release()

def has_cycle(graph):
    WHITE = 0
    GRAY = 1
    BLACK = 2

    color = {node: WHITE for node in graph}

    def dfs(node):
        color[node] = GRAY

        for nxt in graph[node]:
            if color[nxt] == GRAY:
                return True

            if color[nxt] == WHITE and dfs(nxt):
                return True

        color[node] = BLACK
        return False

    return any(
        color[node] == WHITE and dfs(node)
        for node in graph
    )


wait_for = {
    "A": ["B"],
    "B": ["A"],
}

print(has_cycle(wait_for))  # True

def is_safe(available, allocation, maximum):
    n = len(allocation)
    r = len(available)

    need = [
        [
            maximum[i][j] - allocation[i][j]
            for j in range(r)
        ]
        for i in range(n)
    ]

    work = available.copy()
    finished = [False] * n
    sequence = []

    while len(sequence) < n:
        found = False

        for i in range(n):
            if finished[i]:
                continue

            if all(need[i][j] <= work[j] for j in range(r)):
                # 假定该进程完成，释放已持有资源
                for j in range(r):
                    work[j] += allocation[i][j]

                finished[i] = True
                sequence.append(i)
                found = True
                break

        if not found:
            return False, sequence

    return True, sequence

available = [3]

allocation = [
    [3],  # P0
    [2],  # P1
    [2],  # P2
]

maximum = [
    [7],
    [4],
    [5],
]

safe, sequence = is_safe(
    available, allocation, maximum
)

print(safe)
print(sequence)

True
[1, 0, 2]

available = [0]

allocation = [
    [6],  # P0 新获得3个
    [2],
    [2],
]

safe, sequence = is_safe(
    available, allocation, maximum
)

print(safe)
print(sequence)

False
[]

BEGIN;

UPDATE accounts
SET balance = balance - 100
WHERE account_id = 1;

UPDATE accounts
SET balance = balance + 100
WHERE account_id = 2;

COMMIT;

BEGIN;

UPDATE accounts
SET balance = balance - 50
WHERE account_id = 2;

UPDATE accounts
SET balance = balance + 50
WHERE account_id = 1;

COMMIT;

40P01

1205