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ke/sched: fix lock inversion in scheduler

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Taking next's lock can be a lock inversion with the turnstile code.
Use a switching field instead and spin on that.
This commit is contained in:
Loïc Grégoire
2026-07-19 20:40:50 -04:00
parent ea33fcab30
commit 878be31c7a
4 changed files with 24 additions and 9 deletions
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src/kern/sched.zig
+11 -5
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@@ -355,6 +355,8 @@ pub fn handle_preemption(cpu: *PerCpu) void {
cpu.queues_lock.release_no_ipl();
cur.lock.acquire_no_ipl();
cur.switching.store(true, .monotonic);
if (cur != cpu.idle_thread and cur.state.load(.monotonic) != .Blocked) {
// Put it back in the queue.
cpu.queues_lock.acquire_no_ipl();
@@ -363,8 +365,9 @@ pub fn handle_preemption(cpu: *PerCpu) void {
cpu.queues_lock.release_no_ipl();
}
next.lock.acquire_no_ipl();
next.lock.release_no_ipl();
while (next.switching.load(.monotonic) == true) {
std.atomic.spinLoopHint();
}
do_switch(cpu, cur, next);
@@ -435,6 +438,7 @@ pub fn yield() void {
const td = percpu.local().current_thread.?;
td.lock.acquire_no_ipl();
td.switching.store(true, .monotonic);
const cpu = percpu.local();
if (td != cpu.idle_thread and td.state.load(.monotonic) == .Running) {
@@ -512,14 +516,16 @@ pub fn yield_locked() void {
sched_cpu.queues_lock.release_no_ipl();
if (next != null and cur.? != next.?) {
// Switch into the thread. Take its lock here to ensure it is not still on its stack.
next.?.lock.acquire_no_ipl();
next.?.lock.release_no_ipl();
// Switch into the thread. Ensure it's not switching.
while (next.?.switching.load(.monotonic) == true) {
std.atomic.spinLoopHint();
}
do_switch(sched_cpu, cur.?, next.?);
} else {
// Ensure curthread is not marked as selected.
cur.?.state.store(.Running, .monotonic);
cur.?.switching.store(false, .monotonic);
cur.?.lock.release_no_ipl();
}