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https://github.com/rdmsr/zag.git
synced 2026-10-27 13:58:20 +00:00
ke: optimize mutex
Optimize the mutex code in a few ways: - Split the fast and slow paths into their own functions, inline the fast path. - Raise IPL only when acquiring a turnstile lock. - Avoid taking the turnstile lock if the "waiters" bit is not set. - Do adaptive spinning only when the lock owner is running.
This commit is contained in:
Loïc Grégoire
+170
-70
@@ -1,37 +1,68 @@
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//! Adaptive lock implementation.
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const std = @import("std");
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const rtl = @import("rtl");
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const config = @import("config");
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const std = @import("std");
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const assert = std.debug.assert;
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const ke = @import("root").ke;
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const kep = ke.private;
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/// Number of spins before blocking.
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const optimistic_spins = 100;
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/// Number of spins while the lock's owner is running before blocking.
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// TODO: tune this!
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const optimistic_spins = 4000;
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/// owner | waiters (bit 0)
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const State = packed struct(usize) {
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value: usize = 0,
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const waiters: usize = 1;
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const uncontended: State = @fromBackingInt(0);
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fn from(td: *ke.Thread) State {
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return .{ .value = @intFromPtr(td) };
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}
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inline fn owner(s: State) *ke.Thread {
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return @ptrFromInt(s.value & ~waiters);
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}
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inline fn has_waiters(s: State) bool {
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return s.value & waiters != 0;
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}
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inline fn set_waiters(s: State) State {
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return .{ .value = s.value | waiters };
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}
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inline fn clear_waiters(s: State) State {
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return .{ .value = s.value & ~waiters };
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}
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};
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pub const Mutex = struct {
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/// The thread currently holding the mutex, null if unlocked.
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owner: std.atomic.Value(?*ke.Thread),
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wd: kep.warden.LockData,
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state: std.atomic.Value(State),
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wd: kep.warden.LockData = undefined,
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pub fn init(class: []const u8) Mutex {
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if (config.warden) {
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const cl = kep.warden.find_or_create_lock_class(class);
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return .{
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.owner = std.atomic.Value(?*ke.Thread).init(null),
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.state = std.atomic.Value(State).init(.{}),
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.wd = cl,
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};
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}
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return .{ .owner = std.atomic.Value(?*ke.Thread).init(null) };
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return .{ .state = std.atomic.Value(State).init(.{}) };
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}
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pub fn is_locked(self: *Mutex) bool {
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return self.owner.load(.monotonic) != null;
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return self.state.load(.monotonic) != State.uncontended;
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}
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pub fn acquire(m: *Mutex) void {
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pub inline fn acquire(m: *Mutex) void {
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const curtd = ke.thread.current();
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if (config.warden) {
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@@ -42,91 +73,160 @@ pub const Mutex = struct {
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kep.warden.acquired(m, m.wd, .Lock);
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};
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// Very fast path: the lock is uncontended and we can acquire it
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//immediately.
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const cur_owner = m.owner.cmpxchgStrong(
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null,
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curtd,
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// Fast path: the lock is uncontended and we can acquire it
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// immediately.
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_ = m.state.cmpxchgStrong(
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State.uncontended,
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State.from(curtd),
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.acquire,
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.monotonic,
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);
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) orelse {
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@branchHint(.likely);
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if (cur_owner == null) {
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// We got the lock.
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return;
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}
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};
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// Fast path: Try to acquire without a turnstile by spinning a bit.
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for (0..optimistic_spins) |_| {
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if (m.owner.load(.monotonic) != null) {
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std.atomic.spinLoopHint();
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continue;
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}
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// Nope, go to the slow path.
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m.acquire_slow();
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}
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if (m.owner.cmpxchgWeak(
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null,
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curtd,
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.acquire,
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.monotonic,
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) != null) {
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continue;
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}
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fn acquire_slow(m: *Mutex) void {
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@branchHint(.unlikely);
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return;
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}
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const curtd = ke.thread.current();
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const ipl = ke.ipl.raise(.Dispatch);
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retry: while (true) {
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var state = m.state.load(.monotonic);
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// Slow path: contended, block on a turnstile.
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while (true) {
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const ts = kep.turnstile.lookup(m);
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const owner = m.owner.load(.monotonic);
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if (owner == null) {
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// Lock was released between lookup and here.
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kep.turnstile.exit(m, ts);
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if (m.owner.cmpxchgStrong(
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null,
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curtd,
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if (state == State.uncontended) {
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// The lock seems free, retry acquiring it via CAS.
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if (m.state.cmpxchgStrong(
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State.uncontended,
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State.from(curtd),
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.acquire,
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.monotonic,
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) == null) {
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break;
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}
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continue;
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}
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// Still contended, lock the turnstile chain.
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var ipl = ke.ipl.raise(.Dispatch);
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const chain = kep.turnstile.lock_chain(m);
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state = m.state.load(.monotonic);
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if (state == State.uncontended) {
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// Something changed, retry acquiring.
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kep.turnstile.unlock_chain(chain);
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ke.ipl.lower(ipl);
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continue :retry;
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}
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// Set the waiters bit so that release() will also take the lock.
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waiters: while (!state.has_waiters()) {
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if (state == State.uncontended) {
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kep.turnstile.unlock_chain(chain);
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ke.ipl.lower(ipl);
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continue :retry;
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}
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const marked = state.set_waiters();
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state = m.state.cmpxchgWeak(
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state,
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marked,
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.monotonic,
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.monotonic,
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) orelse {
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state = marked;
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break :waiters;
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};
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}
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// Look up the current owner, there is a guarantee that the thread
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// pointer is still valid while we hold the lock here because
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// upon release, whomever holds the lock right now will notice the
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// waiters bit and try to take the same chain lock.
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const owner = state.owner();
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if (owner.state.load(.monotonic) == .Running) {
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const cpu = owner.cpu orelse unreachable;
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const cpu_curtd = kep.sched.current_thread.remote(cpu);
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// The lock's owner is running, spin adaptively
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// until it stops running or we run out of gas.
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kep.turnstile.unlock_chain(chain);
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ke.ipl.lower(ipl);
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var gas: usize = 0;
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while (@atomicLoad(
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?*ke.Thread,
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cpu_curtd,
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.unordered,
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) == owner and gas < optimistic_spins) : (gas += 1) {
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if (m.state.load(.monotonic) == State.uncontended) {
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// Got released, retry acquiring it.
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continue :retry;
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}
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std.atomic.spinLoopHint();
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}
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// Couldn't get it via adaptive spinning, now actually block
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// on a turnstile.
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ipl = ke.ipl.raise(.Dispatch);
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_ = kep.turnstile.lock_chain(m);
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if (m.state.load(.monotonic) != state) {
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// Could've gotten released in the meantime, try again.
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kep.turnstile.unlock_chain(chain);
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ke.ipl.lower(ipl);
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continue :retry;
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}
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}
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const ts = kep.turnstile.lookup_locked(m, chain);
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if (m.state.load(.monotonic) != state) {
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// Something changed between lookup and here.
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kep.turnstile.exit(m, ts);
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ke.ipl.lower(ipl);
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continue :retry;
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}
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// Block until woken.
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kep.turnstile.block(
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ts,
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m,
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.{ .single = owner orelse unreachable },
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.{ .single = state.owner() },
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.Exclusive,
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);
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// Re-try acquisition after wakeup.
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if (m.owner.cmpxchgStrong(
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null,
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curtd,
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.acquire,
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.monotonic,
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) == null) {
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break;
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}
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}
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ke.ipl.lower(ipl);
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}
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pub fn release(m: *Mutex) void {
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pub inline fn release(m: *Mutex) void {
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const old = m.state.swap(State.uncontended, .release);
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if (old.has_waiters()) {
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m.release_slow();
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}
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if (config.warden) {
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kep.warden.released(m, .Lock);
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}
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}
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/// Called when there were waiters on the lock that we need to wake.
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fn release_slow(m: *Mutex) void {
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@branchHint(.unlikely);
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// Look up the turnstile.
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const ipl = ke.ipl.raise(.Dispatch);
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const ts = kep.turnstile.lookup(m);
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m.owner.store(null, .release);
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defer if (config.warden) {
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kep.warden.released(m, .Lock);
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};
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if (ts == null) {
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kep.turnstile.exit(m, ts);
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ke.ipl.lower(ipl);
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@@ -136,10 +236,10 @@ pub const Mutex = struct {
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var waiters: rtl.List = undefined;
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const turn = ts orelse unreachable;
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// Note: wake up all waiters.
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// This so-called "lock barging" (name from WTF::ParkingLot) has been
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// to be better because this avoids lock convoys,
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// see this mysterious 70s paper:
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// Wake ALL the waiters. This so-called "lock barging" is meant to avoid
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// lock convoys.
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//
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// See this mysterious 70s paper:
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// https://dl.acm.org/doi/pdf/10.1145/850657.850659
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kep.turnstile.signal(
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turn,
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