[patch V4 49/68] locking/ww_mutex: Add RT priority to W/W order

Thomas Gleixner
2021-08-11T12:26:47Z

From: Peter Zijlstra <peterz@infradead.org>

RTmutex based ww_mutexes cannot order based on timestamp. They have to
order based on priority. Add the necessary decision logic.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

---
kernel/locking/ww_mutex.h | 64 +++++++++++++++++++++++++++++++++++-----------
1 file changed, 49 insertions(+), 15 deletions(-)
---
--- a/kernel/locking/ww_mutex.h
+++ b/kernel/locking/ww_mutex.h
@@ -219,19 +219,54 @@ ww_mutex_lock_acquired(struct ww_mutex *
}

/*
- * Determine if context @a is 'after' context @b. IOW, @a is a younger
- * transaction than @b and depending on algorithm either needs to wait for
- * @b or die.
+ * Determine if @a is 'less' than @b. IOW, either @a is a lower priority task
+ * or, when of equal priority, a younger transaction than @b.
+ *
+ * Depending on the algorithm, @a will either need to wait for @b, or die.
*/
static inline bool
-__ww_ctx_stamp_after(struct ww_acquire_ctx *a, struct ww_acquire_ctx *b)
+__ww_ctx_less(struct ww_acquire_ctx *a, struct ww_acquire_ctx *b)
{
+/*
+ * Can only do the RT prio for WW_RT because task->prio isn't stable due to PI,
+ * so the wait_list ordering will go wobbly. rt_mutex re-queues the waiter and
+ * isn't affected by this.
+ */
+#ifdef WW_RT
+ /* kernel prio; less is more */
+ int a_prio = a->task->prio;
+ int b_prio = b->task->prio;
+
+ if (rt_prio(a_prio) || rt_prio(b_prio)) {
+
+ if (a_prio > b_prio)
+ return true;
+
+ if (a_prio < b_prio)
+ return false;
+
+ /* equal static prio */
+
+ if (dl_prio(a_prio)) {
+ if (dl_time_before(b->task->dl.deadline,
+ a->task->dl.deadline))
+ return true;
+
+ if (dl_time_before(a->task->dl.deadline,
+ b->task->dl.deadline))
+ return false;
+ }
+
+ /* equal prio */
+ }
+#endif

+ /* FIFO order tie break -- bigger is younger */
return (signed long)(a->stamp - b->stamp) > 0;
}

/*
- * Wait-Die; wake a younger waiter context (when locks held) such that it can
+ * Wait-Die; wake a lesser waiter context (when locks held) such that it can
* die.
*
* Among waiters with context, only the first one can have other locks acquired
@@ -245,8 +280,7 @@ static bool
if (!ww_ctx->is_wait_die)
return false;

- if (waiter->ww_ctx->acquired > 0 &&
- __ww_ctx_stamp_after(waiter->ww_ctx, ww_ctx)) {
+ if (waiter->ww_ctx->acquired > 0 && __ww_ctx_less(waiter->ww_ctx, ww_ctx)) {
#ifndef WW_RT
debug_mutex_wake_waiter(lock, waiter);
#endif
@@ -257,10 +291,10 @@ static bool
}

/*
- * Wound-Wait; wound a younger @hold_ctx if it holds the lock.
+ * Wound-Wait; wound a lesser @hold_ctx if it holds the lock.
*
- * Wound the lock holder if there are waiters with older transactions than
- * the lock holders. Even if multiple waiters may wound the lock holder,
+ * Wound the lock holder if there are waiters with more important transactions
+ * than the lock holders. Even if multiple waiters may wound the lock holder,
* it's sufficient that only one does.
*/
static bool __ww_mutex_wound(struct MUTEX *lock,
@@ -287,7 +321,7 @@ static bool __ww_mutex_wound(struct MUTE
if (!owner)
return false;

- if (ww_ctx->acquired > 0 && __ww_ctx_stamp_after(hold_ctx, ww_ctx)) {
+ if (ww_ctx->acquired > 0 && __ww_ctx_less(hold_ctx, ww_ctx)) {
hold_ctx->wounded = 1;

/*
@@ -306,8 +340,8 @@ static bool __ww_mutex_wound(struct MUTE
}

/*
- * We just acquired @lock under @ww_ctx, if there are later contexts waiting
- * behind us on the wait-list, check if they need to die, or wound us.
+ * We just acquired @lock under @ww_ctx, if there are more important contexts
+ * waiting behind us on the wait-list, check if they need to die, or wound us.
*
* See __ww_mutex_add_waiter() for the list-order construction; basically the
* list is ordered by stamp, smallest (oldest) first.
@@ -421,7 +455,7 @@ static inline int
return 0;
}

- if (hold_ctx && __ww_ctx_stamp_after(ctx, hold_ctx))
+ if (hold_ctx && __ww_ctx_less(ctx, hold_ctx))
return __ww_mutex_kill(lock, ctx);

/*
@@ -479,7 +513,7 @@ static inline int
if (!cur->ww_ctx)
continue;

- if (__ww_ctx_stamp_after(ww_ctx, cur->ww_ctx)) {
+ if (__ww_ctx_less(ww_ctx, cur->ww_ctx)) {
/*
* Wait-Die: if we find an older context waiting, there
* is no point in queueing behind it, as we'd have to