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

John Stultz
2024-11-25T19:52:10Z

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 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

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

Finally add 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
priority inheritance is done with rt_mutexes.

Cc: Joel Fernandes <joelaf@google.com>
Cc: Qais Yousef <qyousef@layalina.io>
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: Mel Gorman <mgorman@suse.de>
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: Daniel Lezcano <daniel.lezcano@linaro.org>
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 circular blocked_on relationships
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
v8:
* Fix issue leaving task BO_BLOCKED when calling into optimistic
spinning path.
* Include helper to better handle BO_BLOCKED->BO_WAKING transitions
v9:
* Typo fixup pointed out by Metin
* Cleanup BO_WAKING->BO_RUNNABLE transitions for the !proxy case
* Many cleanups and simplifications suggested by Metin
v11:
* Whitespace fixup pointed out by Metin
v13:
* Refactor set_blocked_on helpers clean things up a bit
v14:
* Small build fixup with PREEMPT_RT
---
include/linux/sched.h | 66 ++++++++++++++++++++++++++++++++----
init/init_task.c | 1 +
kernel/fork.c | 4 +--
kernel/locking/mutex-debug.c | 9 ++---
kernel/locking/mutex.c | 40 ++++++++++++++++++----
kernel/locking/ww_mutex.h | 24 +++++++++++--
kernel/sched/core.c | 1 +
7 files changed, 125 insertions(+), 20 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 24e338ac34d7b..0ad8033f8c2b9 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -34,6 +34,7 @@
#include <linux/sched/prio.h>
#include <linux/sched/types.h>
#include <linux/signal_types.h>
+#include <linux/spinlock.h>
#include <linux/syscall_user_dispatch_types.h>
#include <linux/mm_types_task.h>
#include <linux/netdevice_xmit.h>
@@ -775,6 +776,12 @@ struct kmap_ctrl {
#endif
};

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

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);
__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);
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);

--
2.47.0.371.ga323438b13-goog