locking/mutex: Rework task_struct::blocked_on

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

   task
     | blocked-on
     v
   mutex
     | owner
     v
   task

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.

For serialization, blocked-on is only set by the task itself
(current). And both when setting or clearing (potentially by
others), is done while holding the mutex::wait_lock.

[minor changes while rebasing]
[jstultz: Fix blocked_on tracking in __mutex_lock_common in error paths]
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
Signed-off-by: Connor O'Brien <connoro@google.com>
Signed-off-by: John Stultz <jstultz@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Link: https://lkml.kernel.org/r/20250712033407.2383110-3-jstultz@google.com

diff --git a/include/linux/sched.h b/include/linux/sched.h
index f225b6b1baa3558207e0d579706ec59769a63439..33ad240ec9004a1f4cc278437d798ccf08e7dee3 100644 (file)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1230,10 +1230,7 @@ struct task_struct {
        struct rt_mutex_waiter          *pi_blocked_on;
 #endif
 
-#ifdef CONFIG_DEBUG_MUTEXES
-       /* Mutex deadlock detection: */
-       struct mutex_waiter             *blocked_on;
-#endif
+       struct mutex                    *blocked_on;    /* lock we're blocked on */
 
 #ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
        /*

diff --git a/kernel/fork.c b/kernel/fork.c
index 1ee8eb11f38bae1d2eb6de9494aea94b7a19e6c3..5f87f05aff4a0f62c5ef8e2c9e4c9115e4e01e5c 100644 (file)
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2123,9 +2123,8 @@ __latent_entropy struct task_struct *copy_process(
        lockdep_init_task(p);
 #endif
 
-#ifdef CONFIG_DEBUG_MUTEXES
        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 6e6f6071cfa279584683634c291e6e4c91dc90b2..758b7a6792b0cba0ce4651ba4d4c0773662d1a2f 100644 (file)
--- 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(task->blocked_on);
 }
 
 void debug_mutex_remove_waiter(struct mutex *lock, struct mutex_waiter *waiter,
                         struct task_struct *task)
 {
+       struct mutex *blocked_on = READ_ONCE(task->blocked_on);
+
        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 a39ecccbd106af4d6dd008648b94c4c12f769a1a..e2f59863a866e70e05986d0691e49d3269b04fea 100644 (file)
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -644,6 +644,8 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
                        goto err_early_kill;
        }
 
+       WARN_ON(current->blocked_on);
+       current->blocked_on = lock;
        set_current_state(state);
        trace_contention_begin(lock, LCB_F_MUTEX);
        for (;;) {
@@ -680,6 +682,12 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 
                first = __mutex_waiter_is_first(lock, &waiter);
 
+               /*
+                * 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 aquire.
+                */
+               current->blocked_on = lock;
                set_current_state(state);
                /*
                 * Here we order against unlock; we must either see it change
@@ -691,8 +699,11 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 
                if (first) {
                        trace_contention_begin(lock, LCB_F_MUTEX | LCB_F_SPIN);
+                       /* clear blocked_on as mutex_optimistic_spin may schedule() */
+                       current->blocked_on = NULL;
                        if (mutex_optimistic_spin(lock, ww_ctx, &waiter))
                                break;
+                       current->blocked_on = lock;
                        trace_contention_begin(lock, LCB_F_MUTEX);
                }
 
@@ -700,6 +711,7 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
        }
        raw_spin_lock_irqsave(&lock->wait_lock, flags);
 acquired:
+       current->blocked_on = NULL;
        __set_current_state(TASK_RUNNING);
 
        if (ww_ctx) {
@@ -729,9 +741,11 @@ skip_wait:
        return 0;
 
 err:
+       current->blocked_on = 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_irqrestore_wake(&lock->wait_lock, flags, &wake_q);
        debug_mutex_free_waiter(&waiter);
@@ -942,6 +956,14 @@ static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigne
                next = waiter->task;
 
                debug_mutex_wake_waiter(lock, waiter);
+               /*
+                * Unlock wakeups can be happening in parallel
+                * (when optimistic spinners steal and release
+                * the lock), so blocked_on may already be
+                * cleared here.
+                */
+               WARN_ON(next->blocked_on && next->blocked_on != lock);
+               next->blocked_on = NULL;
                wake_q_add(&wake_q, next);
        }
 

diff --git a/kernel/locking/ww_mutex.h b/kernel/locking/ww_mutex.h
index 37f025a096c9d1a3cb78eb6155a35978d4a322ac..45fe05e51db18843da2d991cab319a2e7a70c5b2 100644 (file)
--- a/kernel/locking/ww_mutex.h
+++ b/kernel/locking/ww_mutex.h
@@ -283,7 +283,15 @@ __ww_mutex_die(struct MUTEX *lock, struct MUTEX_WAITER *waiter,
        if (waiter->ww_ctx->acquired > 0 && __ww_ctx_less(waiter->ww_ctx, ww_ctx)) {
 #ifndef WW_RT
                debug_mutex_wake_waiter(lock, waiter);
+               /*
+                * When waking up the task to die, be sure to clear the
+                * blocked_on pointer. Otherwise we can see circular
+                * blocked_on relationships that can't resolve.
+                */
+               WARN_ON(waiter->task->blocked_on &&
+                       waiter->task->blocked_on != lock);
 #endif
+               waiter->task->blocked_on = NULL;
                wake_q_add(wake_q, waiter->task);
        }
 
@@ -331,9 +339,15 @@ 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) {
+                       /*
+                        * When waking up the task to wound, be sure to clear the
+                        * blocked_on pointer. Otherwise we can see circular
+                        * blocked_on relationships that can't resolve.
+                        */
+                       owner->blocked_on = NULL;
                        wake_q_add(wake_q, owner);
-
+               }
                return true;
        }