Deadlock

/* A */ CREATE TABLE deadlock(
          id INT UNSIGNED NOT NULL,
          weight INT UNSIGNED NOT NULL,
          PRIMARY KEY(id)
        ) ENGINE=InnoDB;

/* A */ INSERT INTO deadlock (id, weight) VALUES (1, 0), (2, 0);

/* A */ SET SESSION TRANSACTION ISOLATION LEVEL SERIALIZABLE;
/* B */ SET SESSION TRANSACTION ISOLATION LEVEL SERIALIZABLE;

/* A */ BEGIN;
/* B */ BEGIN;

/* A */ UPDATE deadlock SET weight = 1 WHERE id = 1; /* OK */
/* B */ UPDATE deadlock SET weight = 2 WHERE id = 2; /* OK */

/* A */ UPDATE deadlock SET weight = 1 WHERE id = 2; /* Locked (pending) */
/* B */ UPDATE deadlock SET weight = 2 WHERE id = 1; /* Deadlock, B killed, A succeeds */

/* A */ COMMIT; /* OK */
/* A */ COMMIT; /* OK (but does nothing) */