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