Message-ID: <20241106025656.2326794-3-jstultz@google.com>
Date: Tue,  5 Nov 2024 18:56:42 -0800
From: John Stultz <jstultz@...gle.com>
To: LKML <linux-kernel@...r.kernel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Joel Fernandes <joelaf@...gle.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>, kernel-team@...roid.com, 
	"Connor O'Brien" <connoro@...gle.com>, John Stultz <jstultz@...gle.com>
Subject: [RFC][PATCH v13 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

Also add a blocked_on_state value so we can distinguish when a
task is blocked_on a mutex, but is either blocked, waking up, or
runnable (such that it can try to acquire the lock its blocked
on).

This avoids some of the subtle & racy games where the blocked_on
state gets cleared, only to have it re-added by the
mutex_lock_slowpath call when it tries to acquire the lock on
wakeup

Also add blocked_lock to the task_struct so we can safely
serialize the blocked-on state.

Finally add wrappers that are useful to provide correctness
checks. Folded in from a patch by:
  Valentin Schneider <valentin.schneider@....com>

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.

Cc: Joel Fernandes <joelaf@...gle.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: 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
---
 include/linux/sched.h        | 66 ++++++++++++++++++++++++++++++++----
 init/init_task.c             |  1 +
 kernel/fork.c                |  4 +--
 kernel/locking/mutex-debug.c |  9 ++---
 kernel/locking/mutex.c       | 40 ++++++++++++++++++----
 kernel/locking/ww_mutex.h    | 24 +++++++++++--
 kernel/sched/core.c          |  1 +
 7 files changed, 125 insertions(+), 20 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 2a47228a4808..abe23791de30 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -34,6 +34,7 @@
 #include <linux/sched/prio.h>
 #include <linux/sched/types.h>
 #include <linux/signal_types.h>
+#include <linux/spinlock.h>
 #include <linux/syscall_user_dispatch_types.h>
 #include <linux/mm_types_task.h>
 #include <linux/netdevice_xmit.h>
@@ -775,6 +776,12 @@ struct kmap_ctrl {
 #endif
 };
 
+enum blocked_on_state {
+	BO_RUNNABLE,
+	BO_BLOCKED,
+	BO_WAKING,
+};
+
 struct task_struct {
 #ifdef CONFIG_THREAD_INFO_IN_TASK
 	/*
@@ -1195,10 +1202,9 @@ struct task_struct {
 	struct rt_mutex_waiter		*pi_blocked_on;
 #endif
 
-#ifdef CONFIG_DEBUG_MUTEXES
-	/* Mutex deadlock detection: */
-	struct mutex_waiter		*blocked_on;
-#endif
+	enum blocked_on_state		blocked_on_state;
+	struct mutex			*blocked_on;	/* lock we're blocked on */
+	raw_spinlock_t			blocked_lock;
 
 #ifdef CONFIG_DEBUG_ATOMIC_SLEEP
 	int				non_block_count;
@@ -2116,6 +2122,56 @@ extern int __cond_resched_rwlock_write(rwlock_t *lock);
 	__cond_resched_rwlock_write(lock);					\
 })
 
+static inline void __set_blocked_on_runnable(struct task_struct *p)
+{
+	lockdep_assert_held(&p->blocked_lock);
+
+	if (p->blocked_on_state == BO_WAKING)
+		p->blocked_on_state = BO_RUNNABLE;
+}
+
+static inline void set_blocked_on_runnable(struct task_struct *p)
+{
+	unsigned long flags;
+
+	if (!sched_proxy_exec())
+		return;
+
+	raw_spin_lock_irqsave(&p->blocked_lock, flags);
+	__set_blocked_on_runnable(p);
+	raw_spin_unlock_irqrestore(&p->blocked_lock, flags);
+}
+
+static inline void set_blocked_on_waking(struct task_struct *p)
+{
+	lockdep_assert_held(&p->blocked_lock);
+
+	if (p->blocked_on_state == BO_BLOCKED)
+		p->blocked_on_state = BO_WAKING;
+}
+
+static inline void set_task_blocked_on(struct task_struct *p, struct mutex *m)
+{
+	lockdep_assert_held(&p->blocked_lock);
+
+	/*
+	 * Check we are clearing values to NULL or setting NULL
+	 * to values to ensure we don't overwrite existing mutex
+	 * values or clear already cleared values
+	 */
+	WARN_ON((!m && !p->blocked_on) || (m && p->blocked_on));
+
+	p->blocked_on = m;
+	p->blocked_on_state = m ? BO_BLOCKED : BO_RUNNABLE;
+}
+
+static inline struct mutex *get_task_blocked_on(struct task_struct *p)
+{
+	lockdep_assert_held(&p->blocked_lock);
+
+	return p->blocked_on;
+}
+
 static __always_inline bool need_resched(void)
 {
 	return unlikely(tif_need_resched());
@@ -2155,8 +2211,6 @@ extern bool sched_task_on_rq(struct task_struct *p);
 extern unsigned long get_wchan(struct task_struct *p);
 extern struct task_struct *cpu_curr_snapshot(int cpu);
 
-#include <linux/spinlock.h>
-
 /*
  * In order to reduce various lock holder preemption latencies provide an
  * interface to see if a vCPU is currently running or not.
diff --git a/init/init_task.c b/init/init_task.c
index 136a8231355a..c2da9cce36bd 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -139,6 +139,7 @@ struct task_struct init_task __aligned(L1_CACHE_BYTES) = {
 	.journal_info	= NULL,
 	INIT_CPU_TIMERS(init_task)
 	.pi_lock	= __RAW_SPIN_LOCK_UNLOCKED(init_task.pi_lock),
+	.blocked_lock	= __RAW_SPIN_LOCK_UNLOCKED(init_task.blocked_lock),
 	.timer_slack_ns = 50000, /* 50 usec default slack */
 	.thread_pid	= &init_struct_pid,
 	.thread_node	= LIST_HEAD_INIT(init_signals.thread_head),
diff --git a/kernel/fork.c b/kernel/fork.c
index 7d950e93f080..99ebc8269140 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2229,6 +2229,7 @@ __latent_entropy struct task_struct *copy_process(
 	ftrace_graph_init_task(p);
 
 	rt_mutex_init_task(p);
+	raw_spin_lock_init(&p->blocked_lock);
 
 	lockdep_assert_irqs_enabled();
 #ifdef CONFIG_PROVE_LOCKING
@@ -2326,9 +2327,8 @@ __latent_entropy struct task_struct *copy_process(
 	lockdep_init_task(p);
 #endif
 
-#ifdef CONFIG_DEBUG_MUTEXES
+	p->blocked_on_state = BO_RUNNABLE;
 	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 6e6f6071cfa2..1d8cff71f65e 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(get_task_blocked_on(task));
 }
 
 void debug_mutex_remove_waiter(struct mutex *lock, struct mutex_waiter *waiter,
 			 struct task_struct *task)
 {
+	struct mutex *blocked_on = get_task_blocked_on(task);
+
 	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 3302e52f0c96..8f5d3fe6c102 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -597,6 +597,7 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 	}
 
 	raw_spin_lock_irqsave(&lock->wait_lock, flags);
+	raw_spin_lock(&current->blocked_lock);
 	/*
 	 * After waiting to acquire the wait_lock, try again.
 	 */
@@ -627,6 +628,7 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 			goto err_early_kill;
 	}
 
+	set_task_blocked_on(current, lock);
 	set_current_state(state);
 	trace_contention_begin(lock, LCB_F_MUTEX);
 	for (;;) {
@@ -639,7 +641,7 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 		 * the handoff.
 		 */
 		if (__mutex_trylock(lock))
-			goto acquired;
+			break; /* acquired */;
 
 		/*
 		 * Check for signals and kill conditions while holding
@@ -657,6 +659,7 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 				goto err;
 		}
 
+		raw_spin_unlock(&current->blocked_lock);
 		raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
 		/* Make sure we do wakeups before calling schedule */
 		wake_up_q(&wake_q);
@@ -666,6 +669,13 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 
 		first = __mutex_waiter_is_first(lock, &waiter);
 
+		raw_spin_lock_irqsave(&lock->wait_lock, flags);
+		raw_spin_lock(&current->blocked_lock);
+
+		/*
+		 * Re-set blocked_on_state as unlock path set it to WAKING/RUNNABLE
+		 */
+		current->blocked_on_state = BO_BLOCKED;
 		set_current_state(state);
 		/*
 		 * Here we order against unlock; we must either see it change
@@ -676,16 +686,26 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 			break;
 
 		if (first) {
+			bool opt_acquired;
+
+			/*
+			 * mutex_optimistic_spin() can schedule, so  we need to
+			 * release these locks before calling it.
+			 */
+			current->blocked_on_state = BO_RUNNABLE;
+			raw_spin_unlock(&current->blocked_lock);
+			raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
 			trace_contention_begin(lock, LCB_F_MUTEX | LCB_F_SPIN);
-			if (mutex_optimistic_spin(lock, ww_ctx, &waiter))
+			opt_acquired = mutex_optimistic_spin(lock, ww_ctx, &waiter);
+			raw_spin_lock_irqsave(&lock->wait_lock, flags);
+			raw_spin_lock(&current->blocked_lock);
+			current->blocked_on_state = BO_BLOCKED;
+			if (opt_acquired)
 				break;
 			trace_contention_begin(lock, LCB_F_MUTEX);
 		}
-
-		raw_spin_lock_irqsave(&lock->wait_lock, flags);
 	}
-	raw_spin_lock_irqsave(&lock->wait_lock, flags);
-acquired:
+	set_task_blocked_on(current, NULL);
 	__set_current_state(TASK_RUNNING);
 
 	if (ww_ctx) {
@@ -710,16 +730,20 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 	if (ww_ctx)
 		ww_mutex_lock_acquired(ww, ww_ctx);
 
+	raw_spin_unlock(&current->blocked_lock);
 	raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
 	wake_up_q(&wake_q);
 	preempt_enable();
 	return 0;
 
 err:
+	set_task_blocked_on(current, NULL);
 	__set_current_state(TASK_RUNNING);
 	__mutex_remove_waiter(lock, &waiter);
 err_early_kill:
+	WARN_ON(get_task_blocked_on(current));
 	trace_contention_end(lock, ret);
+	raw_spin_unlock(&current->blocked_lock);
 	raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
 	debug_mutex_free_waiter(&waiter);
 	mutex_release(&lock->dep_map, ip);
@@ -928,8 +952,12 @@ static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigne
 
 		next = waiter->task;
 
+		raw_spin_lock(&next->blocked_lock);
 		debug_mutex_wake_waiter(lock, waiter);
+		WARN_ON(get_task_blocked_on(next) != lock);
+		set_blocked_on_waking(next);
 		wake_q_add(&wake_q, next);
+		raw_spin_unlock(&next->blocked_lock);
 	}
 
 	if (owner & MUTEX_FLAG_HANDOFF)
diff --git a/kernel/locking/ww_mutex.h b/kernel/locking/ww_mutex.h
index 37f025a096c9..d1bec62c6ef0 100644
--- a/kernel/locking/ww_mutex.h
+++ b/kernel/locking/ww_mutex.h
@@ -281,10 +281,21 @@ __ww_mutex_die(struct MUTEX *lock, struct MUTEX_WAITER *waiter,
 		return false;
 
 	if (waiter->ww_ctx->acquired > 0 && __ww_ctx_less(waiter->ww_ctx, ww_ctx)) {
+		/* nested as we should hold current->blocked_lock already */
+		raw_spin_lock_nested(&waiter->task->blocked_lock, SINGLE_DEPTH_NESTING);
 #ifndef WW_RT
 		debug_mutex_wake_waiter(lock, waiter);
 #endif
+		/*
+		 * When waking up the task to die, be sure to set the
+		 * blocked_on_state to WAKING. Otherwise we can see
+		 * circular blocked_on relationships that can't
+		 * resolve.
+		 */
+		WARN_ON(get_task_blocked_on(waiter->task) != lock);
+		set_blocked_on_waking(waiter->task);
 		wake_q_add(wake_q, waiter->task);
+		raw_spin_unlock(&waiter->task->blocked_lock);
 	}
 
 	return true;
@@ -331,9 +342,18 @@ 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) {
+			/* nested as we should hold current->blocked_lock already */
+			raw_spin_lock_nested(&owner->blocked_lock, SINGLE_DEPTH_NESTING);
+			/*
+			 * When waking up the task to wound, be sure to set the
+			 * blocked_on_state flag. Otherwise we can see circular
+			 * blocked_on relationships that can't resolve.
+			 */
+			set_blocked_on_waking(owner);
 			wake_q_add(wake_q, owner);
-
+			raw_spin_unlock(&owner->blocked_lock);
+		}
 		return true;
 	}
 
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 731ebd8614a9..f040feed9df3 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4309,6 +4309,7 @@ int try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
 		ttwu_queue(p, cpu, wake_flags);
 	}
 out:
+	set_blocked_on_runnable(p);
 	if (success)
 		ttwu_stat(p, task_cpu(p), wake_flags);
 
-- 
2.47.0.199.ga7371fff76-goog

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