ke: add turnstile and mutex

src/kern/mutex.zig

@@ -0,0 +1,69 @@
+const std = @import("std");
+const ke = @import("root").ke;
+const ki = ke.private;
+
+pub const Mutex = struct {
+    /// The thread currently holding the mutex, null if unlocked.
+    owner: std.atomic.Value(?*ke.Thread),
+
+    pub fn init() Mutex {
+        return .{ .owner = .init(null) };
+    }
+
+    pub fn acquire(m: *Mutex) void {
+        const ipl = ke.ipl.raise(.Dispatch);
+        const curtd = ki.sched.percpu.local().current_thread.?;
+
+        // Fast path: try to acquire without a turnstile.
+        if (m.owner.cmpxchgStrong(null, curtd, .acquire, .monotonic) == null) {
+            ke.ipl.lower(ipl);
+            return;
+        }
+
+        // Slow path: contended, block on a turnstile.
+        while (true) {
+            const ts = ki.turnstile.lookup(m);
+            const owner = m.owner.load(.monotonic);
+
+            if (owner == null) {
+                // Lock was released between lookup and here.
+                ki.turnstile.exit(m);
+                if (m.owner.cmpxchgStrong(null, curtd, .acquire, .monotonic) == null) {
+                    break;
+                }
+                continue;
+            }
+
+            // Block until woken.
+            ki.turnstile.block(ts, m, owner.?, .Shared);
+
+            // Re-try acquisition after wakeup.
+            if (m.owner.cmpxchgStrong(null, curtd, .acquire, .monotonic) == null) {
+                break;
+            }
+        }
+
+        ke.ipl.lower(ipl);
+    }
+
+    pub fn release(m: *Mutex) void {
+        const ipl = ke.ipl.raise(.Dispatch);
+
+        const ts = ki.turnstile.lookup(m);
+
+        m.owner.store(null, .release);
+
+        if (ts == null) {
+            ki.turnstile.exit(m);
+            ke.ipl.lower(ipl);
+            return;
+        }
+
+        // Note: wake up all waiters. This so-called "lock barging" (name from WTF::ParkingLot)
+        // has been shown to be better because this avoids lock convoys, see this mysterious 70s paper:
+        // https://dl.acm.org/doi/pdf/10.1145/850657.850659
+        ki.turnstile.wakeup(ts.?, .Exlusive, ts.?.waiters, null);
+
+        ke.ipl.lower(ipl);
+    }
+};