| ... | ... | @@ -20,13 +20,14 @@ pub const Lock = struct { |
| 20 | 20 | head: usize = UNLOCKED, |
| 21 | 21 | |
| 22 | 22 | const UNLOCKED = 0; |
| 23 | | const LOCKED = 69; |
| 23 | const LOCKED = 1; |
| 24 | 24 | |
| 25 | 25 | const global_event_loop = Loop.instance orelse |
| 26 | 26 | @compileError("std.event.Lock currently only works with event-based I/O"); |
| 27 | 27 | |
| 28 | 28 | const Waiter = struct { |
| 29 | | next: ?*Waiter, |
| 29 | // forced Waiter alignment to ensure it doesn't clash with LOCKED |
| 30 | next: ?*Waiter align(2), |
| 30 | 31 | tail: *Waiter, |
| 31 | 32 | node: Loop.NextTickNode, |
| 32 | 33 | }; |
| ... | ... | @@ -34,6 +35,14 @@ pub const Lock = struct { |
| 34 | 35 | pub fn acquire(self: *Lock) Held { |
| 35 | 36 | const held = self.mutex.acquire(); |
| 36 | 37 | |
| 38 | // self.head transitions from multiple stages depending on the value: |
| 39 | // UNLOCKED -> LOCKED: |
| 40 | // acquire Lock ownership when theres no waiters |
| 41 | // LOCKED -> <Waiter head ptr>: |
| 42 | // Lock is already owned, enqueue first Waiter |
| 43 | // <head ptr> -> <head ptr>: |
| 44 | // Lock is owned with pending waiters. Push our waiter to the queue. |
| 45 | |
| 37 | 46 | if (self.head == UNLOCKED) { |
| 38 | 47 | self.head = LOCKED; |
| 39 | 48 | held.release(); |
| ... | ... | @@ -47,7 +56,7 @@ pub const Lock = struct { |
| 47 | 56 | const head = switch (self.head) { |
| 48 | 57 | UNLOCKED => unreachable, |
| 49 | 58 | LOCKED => null, |
| 50 | | else => @intToPtr(?*Waiter, self.head), |
| 59 | else => @intToPtr(*Waiter, self.head), |
| 51 | 60 | }; |
| 52 | 61 | |
| 53 | 62 | if (head) |h| { |
| ... | ... | @@ -77,9 +86,17 @@ pub const Lock = struct { |
| 77 | 86 | const held = self.lock.mutex.acquire(); |
| 78 | 87 | defer held.release(); |
| 79 | 88 | |
| 89 | // self.head goes through the reverse transition from acquire(): |
| 90 | // <head ptr> -> <new head ptr>: |
| 91 | // pop a waiter from the queue to give Lock ownership when theres still others pending |
| 92 | // <head ptr> -> LOCKED: |
| 93 | // pop the laster waiter from the queue, while also giving it lock ownership when awaken |
| 94 | // LOCKED -> UNLOCKED: |
| 95 | // last lock owner releases lock while no one else is waiting for it |
| 96 | |
| 80 | 97 | switch (self.lock.head) { |
| 81 | 98 | UNLOCKED => { |
| 82 | | std.debug.panic("Lock unlocked when already unlocked", .{}); |
| 99 | unreachable; // Lock unlocked while unlocking |
| 83 | 100 | }, |
| 84 | 101 | LOCKED => { |
| 85 | 102 | self.lock.head = UNLOCKED; |
kprotty <45520026+kprotty@users.noreply.github.com> 2020-09-28 07:25:51-05:00