locking: Add task::blocked_lock to serialize blocked_on state

So far, we have been able to utilize the mutex::wait_lock
for serializing the blocked_on state, but when we move to
proxying across runqueues, we will need to add more state
and a way to serialize changes to this state in contexts
where we don't hold the mutex::wait_lock.

So introduce the task::blocked_lock, which nests under the
mutex::wait_lock in the locking order, and rework the locking
to use it.

Signed-off-by: John Stultz <jstultz@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: K Prateek Nayak <kprateek.nayak@amd.com>
Link: https://patch.msgid.link/20260324191337.1841376-5-jstultz@google.com

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 5a5d3dbc9cdf332c46a96012fceb611a9e944e53..2eef9bc6daaab614f31a7db808050b6cef184987 100644 (file)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1238,6 +1238,7 @@ struct task_struct {
 #endif
 
        struct mutex                    *blocked_on;    /* lock we're blocked on */
+       raw_spinlock_t                  blocked_lock;
 
 #ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
        /*
@@ -2181,57 +2182,42 @@ extern int __cond_resched_rwlock_write(rwlock_t *lock) __must_hold(lock);
 #ifndef CONFIG_PREEMPT_RT
 static inline struct mutex *__get_task_blocked_on(struct task_struct *p)
 {
-       struct mutex *m = p->blocked_on;
-
-       if (m)
-               lockdep_assert_held_once(&m->wait_lock);
-       return m;
+       lockdep_assert_held_once(&p->blocked_lock);
+       return p->blocked_on;
 }
 
 static inline void __set_task_blocked_on(struct task_struct *p, struct mutex *m)
 {
-       struct mutex *blocked_on = READ_ONCE(p->blocked_on);
-
        WARN_ON_ONCE(!m);
        /* The task should only be setting itself as blocked */
        WARN_ON_ONCE(p != current);
-       /* Currently we serialize blocked_on under the mutex::wait_lock */
-       lockdep_assert_held_once(&m->wait_lock);
+       /* Currently we serialize blocked_on under the task::blocked_lock */
+       lockdep_assert_held_once(&p->blocked_lock);
        /*
         * Check ensure we don't overwrite existing mutex value
         * with a different mutex. Note, setting it to the same
         * lock repeatedly is ok.
         */
-       WARN_ON_ONCE(blocked_on && blocked_on != m);
-       WRITE_ONCE(p->blocked_on, m);
-}
-
-static inline void set_task_blocked_on(struct task_struct *p, struct mutex *m)
-{
-       guard(raw_spinlock_irqsave)(&m->wait_lock);
-       __set_task_blocked_on(p, m);
+       WARN_ON_ONCE(p->blocked_on && p->blocked_on != m);
+       p->blocked_on = m;
 }
 
 static inline void __clear_task_blocked_on(struct task_struct *p, struct mutex *m)
 {
-       if (m) {
-               struct mutex *blocked_on = READ_ONCE(p->blocked_on);
-
-               /* Currently we serialize blocked_on under the mutex::wait_lock */
-               lockdep_assert_held_once(&m->wait_lock);
-               /*
-                * There may be cases where we re-clear already cleared
-                * blocked_on relationships, but make sure we are not
-                * clearing the relationship with a different lock.
-                */
-               WARN_ON_ONCE(blocked_on && blocked_on != m);
-       }
-       WRITE_ONCE(p->blocked_on, NULL);
+       /* Currently we serialize blocked_on under the task::blocked_lock */
+       lockdep_assert_held_once(&p->blocked_lock);
+       /*
+        * There may be cases where we re-clear already cleared
+        * blocked_on relationships, but make sure we are not
+        * clearing the relationship with a different lock.
+        */
+       WARN_ON_ONCE(m && p->blocked_on && p->blocked_on != m);
+       p->blocked_on = NULL;
 }
 
 static inline void clear_task_blocked_on(struct task_struct *p, struct mutex *m)
 {
-       guard(raw_spinlock_irqsave)(&m->wait_lock);
+       guard(raw_spinlock_irqsave)(&p->blocked_lock);
        __clear_task_blocked_on(p, m);
 }
 #else

diff --git a/init/init_task.c b/init/init_task.c
index 5c838757fc10eb769c263025da088e16af48e336..b5f48ebdc2b6eb21a61cc901f5b2b865865a4662 100644 (file)
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -169,6 +169,7 @@ 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 bc2bf58b93b6524ca8510474634c7e921ac25f9a..079802cb61002ab43d89e0efee663fc93f1387f9 100644 (file)
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2076,6 +2076,7 @@ __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

diff --git a/kernel/locking/mutex-debug.c b/kernel/locking/mutex-debug.c
index 2c6b02d4699be705ec85526c8b0826153342bac1..cc6aa9c6e9813b2c5d7ad893e1ae7b9321a96470 100644 (file)
--- a/kernel/locking/mutex-debug.c
+++ b/kernel/locking/mutex-debug.c
@@ -54,13 +54,13 @@ void debug_mutex_add_waiter(struct mutex *lock, struct mutex_waiter *waiter,
        lockdep_assert_held(&lock->wait_lock);
 
        /* Current thread can't be already blocked (since it's executing!) */
-       DEBUG_LOCKS_WARN_ON(__get_task_blocked_on(task));
+       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);
+       struct mutex *blocked_on = get_task_blocked_on(task);
 
        DEBUG_LOCKS_WARN_ON(list_empty(&waiter->list));
        DEBUG_LOCKS_WARN_ON(waiter->task != task);

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 2a1d165b3167e25e57ac08f988a2705141f1a2b4..4aa79bcab08c771324b99d23483199577c6e92dc 100644 (file)
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -656,6 +656,7 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
                        goto err_early_kill;
        }
 
+       raw_spin_lock(&current->blocked_lock);
        __set_task_blocked_on(current, lock);
        set_current_state(state);
        trace_contention_begin(lock, LCB_F_MUTEX);
@@ -669,8 +670,9 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
                 * the handoff.
                 */
                if (__mutex_trylock(lock))
-                       goto acquired;
+                       break;
 
+               raw_spin_unlock(&current->blocked_lock);
                /*
                 * Check for signals and kill conditions while holding
                 * wait_lock. This ensures the lock cancellation is ordered
@@ -693,12 +695,14 @@ __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);
                /*
                 * 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 acquire.
                 */
-               set_task_blocked_on(current, lock);
+               __set_task_blocked_on(current, lock);
                set_current_state(state);
                /*
                 * Here we order against unlock; we must either see it change
@@ -709,25 +713,33 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
                        break;
 
                if (first) {
-                       trace_contention_begin(lock, LCB_F_MUTEX | LCB_F_SPIN);
+                       bool opt_acquired;
+
                        /*
                         * mutex_optimistic_spin() can call schedule(), so
-                        * clear blocked on so we don't become unselectable
+                        * we need to release these locks before calling it,
+                        * and clear blocked on so we don't become unselectable
                         * to run.
                         */
-                       clear_task_blocked_on(current, lock);
-                       if (mutex_optimistic_spin(lock, ww_ctx, &waiter))
+                       __clear_task_blocked_on(current, lock);
+                       raw_spin_unlock(&current->blocked_lock);
+                       raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
+
+                       trace_contention_begin(lock, LCB_F_MUTEX | LCB_F_SPIN);
+                       opt_acquired = mutex_optimistic_spin(lock, ww_ctx, &waiter);
+
+                       raw_spin_lock_irqsave(&lock->wait_lock, flags);
+                       raw_spin_lock(&current->blocked_lock);
+                       __set_task_blocked_on(current, lock);
+
+                       if (opt_acquired)
                                break;
-                       set_task_blocked_on(current, lock);
                        trace_contention_begin(lock, LCB_F_MUTEX);
                }
-
-               raw_spin_lock_irqsave(&lock->wait_lock, flags);
        }
-       raw_spin_lock_irqsave(&lock->wait_lock, flags);
-acquired:
        __clear_task_blocked_on(current, lock);
        __set_current_state(TASK_RUNNING);
+       raw_spin_unlock(&current->blocked_lock);
 
        if (ww_ctx) {
                /*
@@ -756,11 +768,11 @@ skip_wait:
        return 0;
 
 err:
-       __clear_task_blocked_on(current, lock);
+       clear_task_blocked_on(current, lock);
        __set_current_state(TASK_RUNNING);
        __mutex_remove_waiter(lock, &waiter);
 err_early_kill:
-       WARN_ON(__get_task_blocked_on(current));
+       WARN_ON(get_task_blocked_on(current));
        trace_contention_end(lock, ret);
        raw_spin_unlock_irqrestore_wake(&lock->wait_lock, flags, &wake_q);
        debug_mutex_free_waiter(&waiter);
@@ -971,7 +983,7 @@ static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigne
                next = waiter->task;
 
                debug_mutex_wake_waiter(lock, waiter);
-               __clear_task_blocked_on(next, lock);
+               clear_task_blocked_on(next, lock);
                wake_q_add(&wake_q, next);
        }
 

diff --git a/kernel/locking/mutex.h b/kernel/locking/mutex.h
index 9ad4da8cea00458bafb741d30f777fe8ddcb5f5e..7a8ba13fee949ee22d4493882ece41984e2a9dd4 100644 (file)
--- a/kernel/locking/mutex.h
+++ b/kernel/locking/mutex.h
@@ -47,6 +47,12 @@ static inline struct task_struct *__mutex_owner(struct mutex *lock)
        return (struct task_struct *)(atomic_long_read(&lock->owner) & ~MUTEX_FLAGS);
 }
 
+static inline struct mutex *get_task_blocked_on(struct task_struct *p)
+{
+       guard(raw_spinlock_irqsave)(&p->blocked_lock);
+       return __get_task_blocked_on(p);
+}
+
 #ifdef CONFIG_DEBUG_MUTEXES
 extern void debug_mutex_lock_common(struct mutex *lock,
                                    struct mutex_waiter *waiter);

diff --git a/kernel/locking/ww_mutex.h b/kernel/locking/ww_mutex.h
index 31a785afee6c0ebfa36a2ceecc76e8fd27c9a739..e4a81790ea7ddd3d11a96e7a207378b307a8f9dc 100644 (file)
--- a/kernel/locking/ww_mutex.h
+++ b/kernel/locking/ww_mutex.h
@@ -289,7 +289,7 @@ __ww_mutex_die(struct MUTEX *lock, struct MUTEX_WAITER *waiter,
                 * blocked_on pointer. Otherwise we can see circular
                 * blocked_on relationships that can't resolve.
                 */
-               __clear_task_blocked_on(waiter->task, lock);
+               clear_task_blocked_on(waiter->task, lock);
                wake_q_add(wake_q, waiter->task);
        }
 
@@ -347,7 +347,7 @@ static bool __ww_mutex_wound(struct MUTEX *lock,
                         * are waking the mutex owner, who may be currently
                         * blocked on a different mutex.
                         */
-                       __clear_task_blocked_on(owner, NULL);
+                       clear_task_blocked_on(owner, NULL);
                        wake_q_add(wake_q, owner);
                }
                return true;

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5b7f378af0422a215c6207d298399dcd63d6b108..1913dbc68eb98360f8095c15188a678a79603d55 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6584,6 +6584,7 @@ static struct task_struct *proxy_deactivate(struct rq *rq, struct task_struct *d
  *   p->pi_lock
  *     rq->lock
  *       mutex->wait_lock
+ *         p->blocked_lock
  *
  * Returns the task that is going to be used as execution context (the one
  * that is actually going to be run on cpu_of(rq)).
@@ -6603,8 +6604,9 @@ find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
                 * and ensure @owner sticks around.
                 */
                guard(raw_spinlock)(&mutex->wait_lock);
+               guard(raw_spinlock)(&p->blocked_lock);
 
-               /* Check again that p is blocked with wait_lock held */
+               /* Check again that p is blocked with blocked_lock held */
                if (mutex != __get_task_blocked_on(p)) {
                        /*
                         * Something changed in the blocked_on chain and