Serializing Tokens - Serializing Tokens in Action

Serializing Tokens in Action

To show how serializing tokens actually work, let's see some pseudocode and what's going on behind the scenes.

Example PseudoCode using Serializing Tokens
Thread A Thread B Behind the Scenes
lwkt_gettoken(T1); iter = list1.head; ... lwkt_gettoken(T1); // blocks // waiting for token T1 A acquires token T1 and uses it to get synchronized access to list1, which is shared by both threads.
lwkt_gettoken(T2); // blocks // waiting for token T1 A's call to lwkt_gettoken(T2) is a blocking function, so A goes to sleep and temporarily loses its tokens. It will be awakened when the scheduler sees that both T1 and T2 are available.
// waiting for T1 and T2 list1.head = list1.head.next; lwkt_releasetoken(T1); B acquires T1 and modifies list1. Note that A's "iter" still points to the old head of the list.
// get the new version of the head: iter = list1.head; // make new list: while (iter != null) { list2.tail = iter; iter = iter.next; } lwkt_releasetoken(T1); lwkt_releasetoken(T2); The scheduler sees that both T1 and T2 are available, so it wakes up thread A. Since A was coded correctly, it refreshes its iterator with the new head of list1, and does some nonblocking operations on it. Note that it would have been better form for A to simply ask for both tokens at the start.

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