Re: [RFC][PATCH v14 2/7] locking/mutex: Rework task_struct::blocked_on

Peter Zijlstra
2024-12-13T23:22:33Z

On Mon, Nov 25, 2024 at 11:51:56AM -0800, John Stultz wrote:

> 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

If you can remember those sublte cases, I'm sure our future selves
would've loved it if you wrote a comment to go with these states :-)

> +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;
> @@ -2118,6 +2124,56 @@ 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);

Do we want to make this:

guard(raw_spinlock_irqsave)(&p->blocked_lock);

?

> + __set_blocked_on_runnable(p);
> + raw_spin_unlock_irqrestore(&p->blocked_lock, flags);
> +}
> +
> +static inline void set_blocked_on_waking(struct task_struct *p)

consistent naming would be __set_blocked_on_waking()

> +{
> + 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)

__set_task_blocked_on()

> +{
> + 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)

__get_task_blocked_on()

> +{
> + lockdep_assert_held(&p->blocked_lock);
> +
> + return p->blocked_on;
> +}

That gets us the __ prefix if the caller is assumed to have taken
blocked_lock.

> static __always_inline bool need_resched(void)
> {
> return unlikely(tif_need_resched());
> @@ -2157,8 +2213,6 @@ 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 e557f622bd906..7e29d86153d9f 100644
> --- a/init/init_task.c
> +++ b/init/init_task.c
> @@ -140,6 +140,7 @@ 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 f253e81d0c28e..160bead843afb 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2231,6 +2231,7 @@ __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
> @@ -2329,9 +2330,8 @@ __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 6e6f6071cfa27..1d8cff71f65e1 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 3302e52f0c967..8f5d3fe6c1029 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);

blocked_on_state mirrors task-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);

And blocked_on_state again mirrors taks-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);

I'm confused -- didn't I kill optimistic spinning for proxy? IIRC it
fundamentally doesn't make sense since we do a hand-off to the donor
thread.

> + 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);

And again..

>
> 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);

and one again..

> __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);

and more..

> 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 37f025a096c9d..d9ff2022eef6f 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);
> + /*
> + * 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);
> #endif
> + 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 d712e177d3b75..f8714050b6d0d 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4350,6 +4350,7 @@ 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);
>

All in all I'm properly confused by this patch... please write
more/better comments changelog.