evl/mutex: extend check on in-band switch

Checking for PI/PP boosting mutex is not enough when dropping to
in-band context: owning any mutex in this case would be wrong, since
this would create a priority inversion.

Extend the logic of evl_detect_boost_drop() to encompass any owned
mutex, renaming it to evl_check_no_mutex() for consistency. As a
side-effect, the thread which attempts to switch in-band while owning
mutex(es) now receives a single HMDIAG_LKDEPEND notification, instead
of notifying all waiter(s) sleeping on those mutexes.

As a consequence, we can drop detect_inband_owner() which becomes
redundant as it detects the same issue from the converse side without
extending the test coverage (i.e. a contender would check whether the
mutex owner is running in-band).

This change does affect the behavior for applications turning on
T_WOLI on waiter threads explicitly. This said, the same issue would
still be detected if CONFIG_EVL_DEBUG_WOLI is set globally though,
which is the recommended configuration during the development stage.

This change also solves an ABBA issue which existed in the former
implementation:

[   40.976962] ======================================================
[   40.976964] WARNING: possible circular locking dependency detected
[   40.976965] 5.15.77-00716-g8390add2f766 torvalds#156 Not tainted
[   40.976968] ------------------------------------------------------
[   40.976969] monitor-pp-lazy/363 is trying to acquire lock:
[   40.976971] ffff99c5c14e5588 (test363.0){....}-{0:0}, at: evl_detect_boost_drop+0x80/0x200
[   40.976987]
[   40.976987] but task is already holding lock:
[   40.976988] ffff99c5c243d818 (monitor-pp-lazy:363){....}-{0:0}, at: evl_detect_boost_drop+0x0/0x200
[   40.976996]
[   40.976996] which lock already depends on the new lock.
[   40.976996]
[   40.976997]
[   40.976997] the existing dependency chain (in reverse order) is:
[   40.976998]
[   40.976998] -> #1 (monitor-pp-lazy:363){....}-{0:0}:
[   40.977003]        fast_grab_mutex+0xca/0x150
[   40.977006]        evl_lock_mutex_timeout+0x60/0xa90
[   40.977009]        monitor_oob_ioctl+0x226/0xed0
[   40.977014]        EVL_ioctl+0x41/0xa0
[   40.977017]        handle_pipelined_syscall+0x3d8/0x490
[   40.977021]        __pipeline_syscall+0xcc/0x2e0
[   40.977026]        pipeline_syscall+0x47/0x120
[   40.977030]        syscall_enter_from_user_mode+0x40/0xa0
[   40.977036]        do_syscall_64+0x15/0xf0
[   40.977039]        entry_SYSCALL_64_after_hwframe+0x61/0xcb
[   40.977044]
[   40.977044] -> #0 (test363.0){....}-{0:0}:
[   40.977048]        __lock_acquire+0x133a/0x2530
[   40.977053]        lock_acquire+0xce/0x2d0
[   40.977056]        evl_detect_boost_drop+0xb0/0x200
[   40.977059]        evl_switch_inband+0x41e/0x540
[   40.977064]        do_oob_syscall+0x1bc/0x3d0
[   40.977067]        handle_pipelined_syscall+0xbe/0x490
[   40.977071]        __pipeline_syscall+0xcc/0x2e0
[   40.977075]        pipeline_syscall+0x47/0x120
[   40.977079]        syscall_enter_from_user_mode+0x40/0xa0
[   40.977083]        do_syscall_64+0x15/0xf0
[   40.977086]        entry_SYSCALL_64_after_hwframe+0x61/0xcb
[   40.977090]
[   40.977090] other info that might help us debug this:
[   40.977090]
[   40.977091]  Possible unsafe locking scenario:
[   40.977091]
[   40.977092]        CPU0                    CPU1
[   40.977093]        ----                    ----
[   40.977094]   lock(monitor-pp-lazy:363);
[   40.977096]                                lock(test363.0);
[   40.977098]                                lock(monitor-pp-lazy:363);
[   40.977100]   lock(test363.0);
[   40.977102]
[   40.977102]  *** DEADLOCK ***
[   40.977102]
[   40.977103] 1 lock held by monitor-pp-lazy/363:
[   40.977105]  #0: ffff99c5c243d818 (monitor-pp-lazy:363){....}-{0:0}, at: evl_detect_boost_drop+0x0/0x200
[   40.977113]

Signed-off-by: Philippe Gerum <rpm@xenomai.org>

include/evl/mutex.h

@@ -80,7 +80,7 @@ void evl_flush_mutex(struct evl_mutex *mutex,
 
 void evl_commit_mutex_ceiling(struct evl_mutex *mutex);
 
-void evl_detect_boost_drop(void);
+void evl_check_no_mutex(void);
 
 void evl_requeue_mutex_wait(struct evl_wait_channel *wchan,
 			struct evl_thread *waiter);

kernel/evl/mutex.c

@@ -20,9 +20,6 @@
 #define for_each_evl_mutex_waiter(__pos, __mutex)			\
 	list_for_each_entry(__pos, &(__mutex)->wchan.wait_list, wait_next)
 
-#define for_each_evl_booster(__pos, __thread)			\
-	list_for_each_entry(__pos, &(__thread)->boosters, next_booster)
-
 static inline int get_ceiling_value(struct evl_mutex *mutex)
 {
 	/*
@@ -409,67 +406,29 @@ static void drop_booster(struct evl_mutex *mutex)
 }
 
 /*
- * Detect when current which is running out-of-band is about to sleep
- * on a mutex currently owned by another thread running in-band.
- *
- * mutex->wchan.lock held, irqs off, curr == this_evl_rq()->curr.
- */
-static void detect_inband_owner(struct evl_mutex *mutex,
-				struct evl_thread *curr)
-{
-	struct evl_thread *owner = mutex->wchan.owner;
-
-	/*
-	 * @curr == this_evl_rq()->curr so no need to grab
-	 * @curr->lock.
-	 */
-	raw_spin_lock(&curr->rq->lock);
-
-	if (curr->info & T_PIALERT) {
-		curr->info &= ~T_PIALERT;
-	} else if (owner->state & T_INBAND) {
-		curr->info |= T_PIALERT;
-		raw_spin_unlock(&curr->rq->lock);
-		evl_notify_thread(curr, EVL_HMDIAG_LKDEPEND, evl_nil);
-		return;
-	}
-
-	raw_spin_unlock(&curr->rq->lock);
-}
-
-/*
- * Detect when current is about to switch in-band while holding a
- * mutex which is causing an active PI or PP boost. Since such a
- * dependency on in-band would cause a priority inversion for the
- * waiter(s), the latter is sent a HM notification if T_WOLI is set.
+ * Detect when current is about to switch in-band while owning a
+ * mutex, which is plain wrong since this would create a priority
+ * inversion. T_WOLI is set for current.
  */
-void evl_detect_boost_drop(void)
+void evl_check_no_mutex(void)
 {
 	struct evl_thread *curr = evl_current();
-	struct evl_thread *waiter;
-	struct evl_mutex *mutex;
 	unsigned long flags;
+	bool notify;
 
 	raw_spin_lock_irqsave(&curr->lock, flags);
-
-	/*
-	 * Iterate over waiters of each mutex we got boosted for due
-	 * to PI/PP.
-	 */
-	for_each_evl_booster(mutex, curr) {
-		raw_spin_lock(&mutex->wchan.lock);
-		for_each_evl_mutex_waiter(waiter, mutex) {
-			if (!(waiter->state & (T_WOLI|T_PIALERT)))
-				continue;
-			raw_spin_lock(&waiter->rq->lock);
-			waiter->info |= T_PIALERT;
-			raw_spin_unlock(&waiter->rq->lock);
-			evl_notify_thread(waiter, EVL_HMDIAG_LKDEPEND, evl_nil);
+	if (!(curr->info & T_PIALERT)) {
+		notify = !list_empty(&curr->owned_mutexes);
+		if (notify) {
+			raw_spin_lock(&curr->rq->lock);
+			curr->info |= T_PIALERT;
+			raw_spin_unlock(&curr->rq->lock);
 		}
-		raw_spin_unlock(&mutex->wchan.lock);
 	}
-
 	raw_spin_unlock_irqrestore(&curr->lock, flags);
+
+	if (notify)
+		evl_notify_thread(curr, EVL_HMDIAG_LKDEPEND, evl_nil);
 }
 
 void __evl_init_mutex(struct evl_mutex *mutex,
@@ -707,9 +666,6 @@ int evl_lock_mutex_timeout(struct evl_mutex *mutex, ktime_t timeout,
 		evl_put_element(&owner->element);
 	}
 
-	if (unlikely(curr->state & T_WOLI))
-		detect_inband_owner(mutex, curr);
-
 	evl_double_thread_lock(curr, owner);
 
 	walk_mode = evl_pi_check;

kernel/evl/sched/core.c

@@ -1292,7 +1292,7 @@ void evl_switch_inband(int cause)
 
 		/* May check for locking inconsistency too. */
 		if (curr->state & T_WOLI)
-			evl_detect_boost_drop();
+			evl_check_no_mutex();
 	}
 
 	/* @curr is now running inband. */