Message-ID: <20250312221147.1865364-3-jstultz@google.com>
Date: Wed, 12 Mar 2025 15:11:32 -0700
From: John Stultz <jstultz@...gle.com>
To: LKML <linux-kernel@...r.kernel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Joel Fernandes <joelagnelf@...dia.com>, 
	Qais Yousef <qyousef@...alina.io>, Ingo Molnar <mingo@...hat.com>, 
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, Valentin Schneider <vschneid@...hat.com>, 
	Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, 
	Zimuzo Ezeozue <zezeozue@...gle.com>, Mel Gorman <mgorman@...e.de>, Will Deacon <will@...nel.org>, 
	Waiman Long <longman@...hat.com>, Boqun Feng <boqun.feng@...il.com>, 
	"Paul E. McKenney" <paulmck@...nel.org>, Metin Kaya <Metin.Kaya@....com>, 
	Xuewen Yan <xuewen.yan94@...il.com>, K Prateek Nayak <kprateek.nayak@....com>, 
	Thomas Gleixner <tglx@...utronix.de>, Daniel Lezcano <daniel.lezcano@...aro.org>, 
	Suleiman Souhlal <suleiman@...gle.com>, kernel-team@...roid.com, 
	"Connor O'Brien" <connoro@...gle.com>, John Stultz <jstultz@...gle.com>
Subject: [RFC PATCH v15 2/7] locking/mutex: Rework task_struct::blocked_on

From: Peter Zijlstra <peterz@...radead.org>

Track the blocked-on relation for mutexes, to allow following this
relation at schedule time.

   task
     | blocked-on
     v
   mutex
     | owner
     v
   task

This all will be used for tracking blocked-task/mutex chains
with the prox-execution patch in a similar fashion to how
priority inheritance is done with rt_mutexes.

For serialization, blocked-on is only set by the task itself
(current). And both when setting or clearing (potentially by
others), is done while holding the mutex::wait_lock.

Cc: Joel Fernandes <joelagnelf@...dia.com>
Cc: Qais Yousef <qyousef@...alina.io>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Juri Lelli <juri.lelli@...hat.com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: Valentin Schneider <vschneid@...hat.com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Ben Segall <bsegall@...gle.com>
Cc: Zimuzo Ezeozue <zezeozue@...gle.com>
Cc: Mel Gorman <mgorman@...e.de>
Cc: Will Deacon <will@...nel.org>
Cc: Waiman Long <longman@...hat.com>
Cc: Boqun Feng <boqun.feng@...il.com>
Cc: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Metin Kaya <Metin.Kaya@....com>
Cc: Xuewen Yan <xuewen.yan94@...il.com>
Cc: K Prateek Nayak <kprateek.nayak@....com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Daniel Lezcano <daniel.lezcano@...aro.org>
Cc: Suleiman Souhlal <suleiman@...gle.com>
Cc: kernel-team@...roid.com
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
[minor changes while rebasing]
Signed-off-by: Juri Lelli <juri.lelli@...hat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Connor O'Brien <connoro@...gle.com>
[jstultz: Fix blocked_on tracking in __mutex_lock_common in error paths]
Signed-off-by: John Stultz <jstultz@...gle.com>
---
v2:
* Fixed blocked_on tracking in error paths that was causing crashes
v4:
* Ensure we clear blocked_on when waking ww_mutexes to die or wound.
  This is critical so we don't get circular blocked_on relationships
  that can't be resolved.
v5:
* Fix potential bug where the skip_wait path might clear blocked_on
  when that path never set it
* Slight tweaks to where we set blocked_on to make it consistent,
  along with extra WARN_ON correctness checking
* Minor comment changes
v7:
* Minor commit message change suggested by Metin Kaya
* Fix WARN_ON conditionals in unlock path (as blocked_on might already
  be cleared), found while looking at issue Metin Kaya raised.
* Minor tweaks to be consistent in what we do under the
  blocked_on lock, also tweaked variable name to avoid confusion
  with label, and comment typos, as suggested by Metin Kaya
* Minor tweak for CONFIG_SCHED_PROXY_EXEC name change
* Moved unused block of code to later in the series, as suggested
  by Metin Kaya
* Switch to a tri-state to be able to distinguish from waking and
  runnable so we can later safely do return migration from ttwu
* Folded together with related blocked_on changes
v8:
* Fix issue leaving task BO_BLOCKED when calling into optimistic
  spinning path.
* Include helper to better handle BO_BLOCKED->BO_WAKING transitions
v9:
* Typo fixup pointed out by Metin
* Cleanup BO_WAKING->BO_RUNNABLE transitions for the !proxy case
* Many cleanups and simplifications suggested by Metin
v11:
* Whitespace fixup pointed out by Metin
v13:
* Refactor set_blocked_on helpers clean things up a bit
v14:
* Small build fixup with PREEMPT_RT
v15:
* Improve consistency of names for functions that assume blocked_lock
  is held, as suggested by Peter
* Use guard instead of separate spinlock/unlock calls, also suggested
  by Peter
* Drop blocked_on_state tri-state for now, as its not needed until
  later in the series, when we get to proxy-migration and return-
  migration.
---
 include/linux/sched.h        |  5 +----
 kernel/fork.c                |  3 +--
 kernel/locking/mutex-debug.c |  9 +++++----
 kernel/locking/mutex.c       | 19 +++++++++++++++++++
 kernel/locking/ww_mutex.h    | 18 ++++++++++++++++--
 5 files changed, 42 insertions(+), 12 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 1462f2c70aefc..03775c44b7073 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1212,10 +1212,7 @@ struct task_struct {
 	struct rt_mutex_waiter		*pi_blocked_on;
 #endif
 
-#ifdef CONFIG_DEBUG_MUTEXES
-	/* Mutex deadlock detection: */
-	struct mutex_waiter		*blocked_on;
-#endif
+	struct mutex			*blocked_on;	/* lock we're blocked on */
 
 #ifdef CONFIG_DEBUG_ATOMIC_SLEEP
 	int				non_block_count;
diff --git a/kernel/fork.c b/kernel/fork.c
index 735405a9c5f32..38f055082d07a 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2357,9 +2357,8 @@ __latent_entropy struct task_struct *copy_process(
 	lockdep_init_task(p);
 #endif
 
-#ifdef CONFIG_DEBUG_MUTEXES
 	p->blocked_on = NULL; /* not blocked yet */
-#endif
+
 #ifdef CONFIG_BCACHE
 	p->sequential_io	= 0;
 	p->sequential_io_avg	= 0;
diff --git a/kernel/locking/mutex-debug.c b/kernel/locking/mutex-debug.c
index 6e6f6071cfa27..758b7a6792b0c 100644
--- a/kernel/locking/mutex-debug.c
+++ b/kernel/locking/mutex-debug.c
@@ -53,17 +53,18 @@ void debug_mutex_add_waiter(struct mutex *lock, struct mutex_waiter *waiter,
 {
 	lockdep_assert_held(&lock->wait_lock);
 
-	/* Mark the current thread as blocked on the lock: */
-	task->blocked_on = waiter;
+	/* Current thread can't be already blocked (since it's executing!) */
+	DEBUG_LOCKS_WARN_ON(task->blocked_on);
 }
 
 void debug_mutex_remove_waiter(struct mutex *lock, struct mutex_waiter *waiter,
 			 struct task_struct *task)
 {
+	struct mutex *blocked_on = READ_ONCE(task->blocked_on);
+
 	DEBUG_LOCKS_WARN_ON(list_empty(&waiter->list));
 	DEBUG_LOCKS_WARN_ON(waiter->task != task);
-	DEBUG_LOCKS_WARN_ON(task->blocked_on != waiter);
-	task->blocked_on = NULL;
+	DEBUG_LOCKS_WARN_ON(blocked_on && blocked_on != lock);
 
 	INIT_LIST_HEAD(&waiter->list);
 	waiter->task = NULL;
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index b36f23de48f1b..37d1966970617 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -627,6 +627,8 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 			goto err_early_kill;
 	}
 
+	WARN_ON(current->blocked_on);
+	current->blocked_on = lock;
 	set_current_state(state);
 	trace_contention_begin(lock, LCB_F_MUTEX);
 	for (;;) {
@@ -663,6 +665,12 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 
 		first = __mutex_waiter_is_first(lock, &waiter);
 
+		/*
+		 * As we likely have been woken up by task
+		 * that has cleared our blocked_on state, re-set
+		 * it to the lock we are trying to aquire.
+		 */
+		current->blocked_on = lock;
 		set_current_state(state);
 		/*
 		 * Here we order against unlock; we must either see it change
@@ -683,6 +691,7 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 	}
 	raw_spin_lock_irqsave(&lock->wait_lock, flags);
 acquired:
+	current->blocked_on = NULL;
 	__set_current_state(TASK_RUNNING);
 
 	if (ww_ctx) {
@@ -712,9 +721,11 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 	return 0;
 
 err:
+	current->blocked_on = NULL;
 	__set_current_state(TASK_RUNNING);
 	__mutex_remove_waiter(lock, &waiter);
 err_early_kill:
+	WARN_ON(current->blocked_on);
 	trace_contention_end(lock, ret);
 	raw_spin_unlock_irqrestore_wake(&lock->wait_lock, flags, &wake_q);
 	debug_mutex_free_waiter(&waiter);
@@ -924,6 +935,14 @@ static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigne
 		next = waiter->task;
 
 		debug_mutex_wake_waiter(lock, waiter);
+		/*
+		 * Unlock wakeups can be happening in parallel
+		 * (when optimistic spinners steal and release
+		 * the lock), so blocked_on may already be
+		 * cleared here.
+		 */
+		WARN_ON(next->blocked_on && next->blocked_on != lock);
+		next->blocked_on = NULL;
 		wake_q_add(&wake_q, next);
 	}
 
diff --git a/kernel/locking/ww_mutex.h b/kernel/locking/ww_mutex.h
index 37f025a096c9d..00db40946328e 100644
--- a/kernel/locking/ww_mutex.h
+++ b/kernel/locking/ww_mutex.h
@@ -284,6 +284,14 @@ __ww_mutex_die(struct MUTEX *lock, struct MUTEX_WAITER *waiter,
 #ifndef WW_RT
 		debug_mutex_wake_waiter(lock, waiter);
 #endif
+		/*
+		 * When waking up the task to die, be sure to clear the
+		 * blocked_on pointer. Otherwise we can see circular
+		 * blocked_on relationships that can't resolve.
+		 */
+		WARN_ON(waiter->task->blocked_on &&
+			waiter->task->blocked_on != lock);
+		waiter->task->blocked_on = NULL;
 		wake_q_add(wake_q, waiter->task);
 	}
 
@@ -331,9 +339,15 @@ static bool __ww_mutex_wound(struct MUTEX *lock,
 		 * it's wounded in __ww_mutex_check_kill() or has a
 		 * wakeup pending to re-read the wounded state.
 		 */
-		if (owner != current)
+		if (owner != current) {
+			/*
+			 * When waking up the task to wound, be sure to clear the
+			 * blocked_on pointer. Otherwise we can see circular
+			 * blocked_on relationships that can't resolve.
+			 */
+			owner->blocked_on = NULL;
 			wake_q_add(wake_q, owner);
-
+		}
 		return true;
 	}
 
-- 
2.49.0.rc0.332.g42c0ae87b1-goog

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