Re: [PATCH v7 05/23] locking/mutex: Rework task_struct::blocked_on

From: Metin Kaya
Date: Thu Dec 21 2023 - 05:13:56 EST

On 20/12/2023 12:18 am, John Stultz wrote:
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

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

task
| blocked-on
v
mutex
| owner
v
task

Also adds a blocked_on_state value so we can distinguqish when a

What about "Also add a blocked_on_state enum to task_struct to distinguish ..." to use an imperative language?
The same for "Also adds" and "Finally adds" below.

task is blocked_on a mutex, but is either blocked, waking up, or
runnable (such that it can try to aquire the lock its blocked

acquire

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 aquire the lock on

ditto

wakeup

wakeup.


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

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

This all will be used for tracking blocked-task/mutex chains
with the prox-execution patch in a similar fashion to how
proxy

priority inheritence is done with rt_mutexes.
inheritance


Cc: Joel Fernandes <joelaf@xxxxxxxxxx>
Cc: Qais Yousef <qyousef@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Cc: Valentin Schneider <vschneid@xxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Ben Segall <bsegall@xxxxxxxxxx>
Cc: Zimuzo Ezeozue <zezeozue@xxxxxxxxxx>
Cc: Youssef Esmat <youssefesmat@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Waiman Long <longman@xxxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
Cc: Metin Kaya <Metin.Kaya@xxxxxxx>
Cc: Xuewen Yan <xuewen.yan94@xxxxxxxxx>
Cc: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: kernel-team@xxxxxxxxxxx
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
[minor changes while rebasing]
Signed-off-by: Juri Lelli <juri.lelli@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Connor O'Brien <connoro@xxxxxxxxxx>
[jstultz: Fix blocked_on tracking in __mutex_lock_common in error paths]
Signed-off-by: John Stultz <jstultz@xxxxxxxxxx>
---
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 ciruclar blocked_on relationships

circular

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
---
include/linux/sched.h | 40 ++++++++++++++++++++++++++++++++----
init/init_task.c | 1 +
kernel/fork.c | 4 ++--
kernel/locking/mutex-debug.c | 9 ++++----
kernel/locking/mutex.c | 35 +++++++++++++++++++++++++++----
kernel/locking/ww_mutex.h | 24 ++++++++++++++++++++--
kernel/sched/core.c | 6 ++++++
7 files changed, 103 insertions(+), 16 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 1e80c330f755..bfe8670f99a1 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -743,6 +743,12 @@ struct kmap_ctrl {
#endif
};
+enum blocked_on_state {
+ BO_RUNNABLE,
+ BO_BLOCKED,
+ BO_WAKING,
+};
+
struct task_struct {
#ifdef CONFIG_THREAD_INFO_IN_TASK
/*
@@ -1149,10 +1155,9 @@ 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;
@@ -2258,6 +2263,33 @@ static inline int rwlock_needbreak(rwlock_t *lock)
#endif
}
+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 exisiting mutex

existing

+ * 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 inline struct mutex *get_task_blocked_on_once(struct task_struct *p)
+{
+ return READ_ONCE(p->blocked_on);
+}

These functions make me think we should use [get, set]_task_blocked_on() for accessing blocked_on & blocked_on_state fields, but there are some references in this patch which we directly access aforementioned fields.
Is this OK?

+
static __always_inline bool need_resched(void)
{
return unlikely(tif_need_resched());
diff --git a/init/init_task.c b/init/init_task.c
index 5727d42149c3..0c31d7d7c7a9 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -131,6 +131,7 @@ struct task_struct init_task
.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 10917c3e1f03..b3ba3d22d8b2 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2358,6 +2358,7 @@ __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
@@ -2456,9 +2457,8 @@ __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 bc8abb8549d2..1eedf7c60c00 100644
--- a/kernel/locking/mutex-debug.c
+++ b/kernel/locking/mutex-debug.c
@@ -52,17 +52,18 @@ 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_once(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 543774506fdb..6084470773f6 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -592,6 +592,7 @@ __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.
*/
@@ -622,6 +623,7 @@ __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 (;;) {
@@ -652,6 +654,7 @@ __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 */
if (!wake_q_empty(&wake_q)) {
@@ -662,6 +665,13 @@ __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
@@ -672,16 +682,25 @@ __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.
+ */
+ 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);
+ 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) {
@@ -706,16 +725,20 @@ __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(current->blocked_on);
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);
@@ -925,8 +948,12 @@ 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(next->blocked_on != lock);
+ next->blocked_on_state = BO_WAKING;
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 9facc0ddfdd3..8dd21ff5eee0 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(waiter->task->blocked_on != lock);
+ waiter->task->blocked_on_state = BO_WAKING;
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.
+ */
+ owner->blocked_on_state = BO_WAKING;
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 a708d225c28e..4e46189d545d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4195,6 +4195,7 @@ bool ttwu_state_match(struct task_struct *p, unsigned int state, int *success)
int try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
{
guard(preempt)();
+ unsigned long flags;
int cpu, success = 0;
if (p == current) {
@@ -4341,6 +4342,11 @@ int try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
ttwu_queue(p, cpu, wake_flags);
}
+ /* XXX can we do something better here for !CONFIG_SCHED_PROXY_EXEC case */

blocked_on* fields are now used even in !CONFIG_SCHED_PROXY_EXEC case.
I'm unsure if we can get rid of lock & unlock lines or entire hunk, but would this be too ugly? I wish we could convert blocked_on_state to an atomic variable.

if (p->blocked_on_state == BO_WAKING) {
raw_spin_lock_irqsave(&p->blocked_lock, flags);
p->blocked_on_state = BO_RUNNABLE;
raw_spin_unlock_irqrestore(&p->blocked_lock, flags);
}

+ raw_spin_lock_irqsave(&p->blocked_lock, flags);
+ if (p->blocked_on_state == BO_WAKING)
+ p->blocked_on_state = BO_RUNNABLE;
+ raw_spin_unlock_irqrestore(&p->blocked_lock, flags);
out:
if (success)
ttwu_stat(p, task_cpu(p), wake_flags);