[PATCH v4 1/2] futex: Provide rt_mutex_.*_schedule() equivalents for futex scheduling
From: Sebastian Andrzej Siewior
Date: Tue Sep 01 2026 - 11:52:40 EST
There is rt_mutex_{pre|post}_schedule() around
rt_mutex_wait_proxy_lock() to ensure that sched_submit_work()/
sched_update_worker() is invoked before we schedule out and block on
rt_mutex while waiting for it become available.
The reason is that blocking on rt_mutex assigns a pi_waiter for the PI
chain and sched_submit_work() will also assign a pi_waiter if it blocks
on lock but a this point we already have a waiter assigned.
We can't skip sched_submit_work() entirely because I/O relies on the
fact that I/O queue is flushed while it blocks on a sleeping lock.
Therefore sched_submit_work() is moved before we block on the lock.
Sleeping lock in this context means mutex or rw_semaphore not spinlock_t
on PREEMPT_RT. Because the mutex abstraction on PREEMPT_RT uses the same
abstraction as the futex proxy lock, the futex code ended up using
rt_mutex_{pre|post}_schedule(), too.
Using it is/ was just to keep the task_struct::sched_rt_mutex assertion
happy. Futex proxy lock is used only in the syscall context of a task.
At this point it never got any I/O that needs to be flushed and it can't
be a workqueue that needs to notify that it will be scheduled out.
Therefore sched_submit_work() does nothing here.
By mistake futex_wait_requeue_pi() -> rt_mutex_wait_proxy_lock() did not
get the rt_mutex_{pre|post}_schedule() annotation. This was not noticed
because in this callchain the lock is (usually) not contended and so
rt_mutex_slowlock_block() does not schedule, triggering the assert.
Adding rt_mutex_pre_schedule() here looks wrong (as noted by PeterZ)
because at this point there is a pi_waiter recorded and invoking
sched_submit_work() with a possible lock contention would be wrong.
Add rt_mutex_futex_{pre|post}_schedule() which toggles the
sched_rt_mutex assert and does not involve sched_submit_work(). Add
asserts here to ensure that sched_submit_work() would do nothing. Use it
only in futex proxy lock case which is rt_mutex_wait_proxy_lock().
Remove it from futex_lock_pi().
Fixes: d14f9e930b90 ("locking/rtmutex: Use rt_mutex specific scheduler helpers")
Reported-by: Yao Kai <yaokai34@xxxxxxxxxx>
Closes: https://lore.kernel.org/all/20260717084922.4153317-2-yaokai34@xxxxxxxxxx
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
include/linux/sched/rt.h | 2 ++
kernel/futex/pi.c | 16 +++-------------
kernel/locking/rtmutex_api.c | 2 ++
kernel/sched/core.c | 16 ++++++++++++++++
4 files changed, 23 insertions(+), 13 deletions(-)
diff --git a/include/linux/sched/rt.h b/include/linux/sched/rt.h
index 4e3338103654c..922935cc33833 100644
--- a/include/linux/sched/rt.h
+++ b/include/linux/sched/rt.h
@@ -52,8 +52,10 @@ static inline bool rt_or_dl_task_policy(struct task_struct *tsk)
#ifdef CONFIG_RT_MUTEXES
extern void rt_mutex_pre_schedule(void);
+extern void rt_mutex_futex_pre_schedule(void);
extern void rt_mutex_schedule(void);
extern void rt_mutex_post_schedule(void);
+extern void rt_mutex_futex_post_schedule(void);
/*
* Must hold either p->pi_lock or task_rq(p)->lock.
diff --git a/kernel/futex/pi.c b/kernel/futex/pi.c
index 88788e584ec8a..98f1b962e59a0 100644
--- a/kernel/futex/pi.c
+++ b/kernel/futex/pi.c
@@ -1070,17 +1070,11 @@ int futex_lock_pi(u32 __user *uaddr, unsigned int flags, ktime_t *time, int tryl
* Caution; releasing @hb in-scope. The hb->lock is still locked
* while the reference is dropped. The reference can not be dropped
* after the unlock because if a user initiated resize is in progress
- * then we might need to wake him. This can not be done after the
- * rt_mutex_pre_schedule() invocation. The hb will remain valid because
- * the thread, performing resize, will block on hb->lock during
- * the requeue.
+ * then we might need to wake him. The hb will remain valid
+ * because the thread, performing resize, will block on
+ * hb->lock during the requeue.
*/
futex_private_hash_put(no_free_ptr(hbr.fph));
- /*
- * Must be done before we enqueue the waiter, here is unfortunately
- * under the hb lock, but that *should* work because it does nothing.
- */
- rt_mutex_pre_schedule();
rt_mutex_init_waiter(&rt_waiter);
@@ -1146,10 +1140,6 @@ int futex_lock_pi(u32 __user *uaddr, unsigned int flags, ktime_t *time, int tryl
* the
*/
futex_q_lockptr_lock(&q);
- /*
- * Waiter is unqueued.
- */
- rt_mutex_post_schedule();
no_block:
/*
* Fixup the pi_state owner and possibly acquire the lock if we
diff --git a/kernel/locking/rtmutex_api.c b/kernel/locking/rtmutex_api.c
index 5d48d64725b12..eb18b094473c9 100644
--- a/kernel/locking/rtmutex_api.c
+++ b/kernel/locking/rtmutex_api.c
@@ -423,6 +423,7 @@ int __sched rt_mutex_wait_proxy_lock(struct rt_mutex_base *lock,
{
int ret;
+ rt_mutex_futex_pre_schedule();
raw_spin_lock_irq(&lock->wait_lock);
/* sleep on the mutex */
set_current_state(TASK_INTERRUPTIBLE);
@@ -433,6 +434,7 @@ int __sched rt_mutex_wait_proxy_lock(struct rt_mutex_base *lock,
*/
fixup_rt_mutex_waiters(lock, true);
raw_spin_unlock_irq(&lock->wait_lock);
+ rt_mutex_futex_post_schedule();
return ret;
}
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index f78275192036b..449ccd871be81 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7637,6 +7637,17 @@ void rt_mutex_pre_schedule(void)
sched_submit_work(current);
}
+/*
+ * Used within the futex syscall context, skips sched_submit_work() because none
+ * its work will be done. Asserts ensure that it is indeed the case.
+ */
+void rt_mutex_futex_pre_schedule(void)
+{
+ lockdep_assert(!(current->flags & (PF_WQ_WORKER | PF_IO_WORKER)));
+ lockdep_assert(!current->plug);
+ lockdep_assert(!fetch_and_set(current->sched_rt_mutex, 1));
+}
+
void rt_mutex_schedule(void)
{
lockdep_assert(current->sched_rt_mutex);
@@ -7649,6 +7660,11 @@ void rt_mutex_post_schedule(void)
lockdep_assert(fetch_and_set(current->sched_rt_mutex, 0));
}
+void rt_mutex_futex_post_schedule(void)
+{
+ lockdep_assert(fetch_and_set(current->sched_rt_mutex, 0));
+}
+
/*
* rt_mutex_setprio - set the current priority of a task
* @p: task to boost
--
2.55.0