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ke: fix livelock in turnstile

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Loïc Grégoire
2026-06-14 11:37:48 -04:00
parent fb1523602d
commit ead1f5b324
1 changed files with 23 additions and 5 deletions
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src/kern/turnstile.zig
+23 -5
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@@ -135,9 +135,11 @@ fn reboost(ts: *Turnstile, to: *ke.Thread) void {
/// successive owner until one is already at least as high.
///
/// Locking:
/// - `root` is held by the caller throughout and never dropped here.
/// - Any other chain bucket is taken with trylock, failure means restart. At
/// most one such bucket is held at a time.
/// - `root` is held by the caller on entry and is held again on return.
/// - If another chain bucket cannot be trylocked, we drop the current thread
/// lock and `root`, then reacquire `root` and restart from `curtd`.
/// - Any other chain bucket is taken with trylock. At most one such bucket is
/// held at a time.
fn propagate(curtd: *ke.Thread, root: *Chain) void {
var donate = curtd.priority;
var thread = curtd;
@@ -150,10 +152,26 @@ fn propagate(curtd: *ke.Thread, root: *Chain) void {
const bucket = &chains[hash_obj(obj)];
// Lock `bucket` and confirm `thread` didn't move before we got it.
// If either fails, restart below.
// If the trylock fails, drop root and restart from curtd.
// If validation fails, restart while still holding root.
const restart = blk: {
if (bucket != root) {
if (!bucket.lock.try_acquire_no_ipl()) break :blk true;
if (!bucket.lock.try_acquire_no_ipl()) {
std.debug.assert(held == null);
// Drop everything and try again. This avoids a livelock
// between two PI walks holding different roots and trying to
// acquire each other.
thread.lock.release_no_ipl();
root.lock.release_no_ipl();
root.lock.acquire_no_ipl();
curtd.lock.acquire_no_ipl();
thread = curtd;
donate = curtd.priority;
continue;
}
held = bucket;
}
break :blk thread.waiting_on != obj;