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

Metin Kaya
2023-12-21T10:13:47Z

On 20/12/2023 12:18 am, John Stultz wrote:
> From: Peter Zijlstra <peterz@infradead.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 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@arm.com>
>
> 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@google.com>
> Cc: Qais Yousef <qyousef@google.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Cc: Valentin Schneider <vschneid@redhat.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Ben Segall <bsegall@google.com>
> Cc: Zimuzo Ezeozue <zezeozue@google.com>
> Cc: Youssef Esmat <youssefesmat@google.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Waiman Long <longman@redhat.com>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> Cc: "Paul E. McKenney" <paulmck@kernel.org>
> Cc: Metin Kaya <Metin.Kaya@arm.com>
> Cc: Xuewen Yan <xuewen.yan94@gmail.com>
> Cc: K Prateek Nayak <kprateek.nayak@amd.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: kernel-team@android.com
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> [minor changes while rebasing]
> Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Signed-off-by: Connor O'Brien <connoro@google.com>
> [jstultz: Fix blocked_on tracking in __mutex_lock_common in error paths]
> Signed-off-by: John Stultz <jstultz@google.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 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);