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address some review changes

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This commit is contained in:
kprotty 2020-09-28 07:25:51 -05:00
parent a31d9f92f2
commit 468a4bf0b4
1 changed files with 21 additions and 4 deletions
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25
lib/std/event/lock.zig
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@ -20,13 +20,14 @@ pub const Lock = struct {
head: usize = UNLOCKED,
const UNLOCKED = 0;
const LOCKED = 69;
const LOCKED = 1;
const global_event_loop = Loop.instance orelse
@compileError("std.event.Lock currently only works with event-based I/O");
const Waiter = struct {
next: ?*Waiter,
// forced Waiter alignment to ensure it doesn't clash with LOCKED
next: ?*Waiter align(2),
tail: *Waiter,
node: Loop.NextTickNode,
};
@ -34,6 +35,14 @@ pub const Lock = struct {
pub fn acquire(self: *Lock) Held {
const held = self.mutex.acquire();
// self.head transitions from multiple stages depending on the value:
// UNLOCKED -> LOCKED:
// acquire Lock ownership when theres no waiters
// LOCKED -> <Waiter head ptr>:
// Lock is already owned, enqueue first Waiter
// <head ptr> -> <head ptr>:
// Lock is owned with pending waiters. Push our waiter to the queue.
if (self.head == UNLOCKED) {
self.head = LOCKED;
held.release();
@ -47,7 +56,7 @@ pub const Lock = struct {
const head = switch (self.head) {
UNLOCKED => unreachable,
LOCKED => null,
else => @intToPtr(?*Waiter, self.head),
else => @intToPtr(*Waiter, self.head),
};
if (head) |h| {
@ -77,9 +86,17 @@ pub const Lock = struct {
const held = self.lock.mutex.acquire();
defer held.release();
// self.head goes through the reverse transition from acquire():
// <head ptr> -> <new head ptr>:
// pop a waiter from the queue to give Lock ownership when theres still others pending
// <head ptr> -> LOCKED:
// pop the laster waiter from the queue, while also giving it lock ownership when awaken
// LOCKED -> UNLOCKED:
// last lock owner releases lock while no one else is waiting for it
switch (self.lock.head) {
UNLOCKED => {
std.debug.panic("Lock unlocked when already unlocked", .{});
unreachable; // Lock unlocked while unlocking
},
LOCKED => {
self.lock.head = UNLOCKED;