Re: [PATCH] softirq: Provide a handshake for canceling tasklets via polling on PREEMPT_RT

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2025-08-19 17:01:07 [+0200], To Tejun Heo wrote:
> Okay. Then let me repost the tasklet patch and make one for workqueue to
> stay in sync.
> I do hope that we end up with a requirement that any kind of teardown
> does not happen from an atomic context ;)

That would be 

------------->8-------------

Subject: [PATCH] workqueue: Provide a handshake for canceling BH workers

While a BH work item is canceled, the core code spins until it
determines that the item completed. On PREEMPT_RT the spinning relies on
a lock in local_bh_disable() to avoid a live lock if the canceling
thread has higher priority than the BH-worker and preempts it. This lock
ensures that the BH-worker makes progress by PI-boosting it.

This lock in local_bh_disable() is a central per-CPU BKL and about to be
removed.

To provide the required synchronisation add a per pool lock. The lock is
acquired by the bh_worker at the begin while the individual callbacks
are invoked. To enforce progress in case of interruption, __flush_work()
needs to acquire the lock.
This will flush all BH-work items assigned to that pool.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 kernel/workqueue.c | 51 ++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 42 insertions(+), 9 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index c6b79b3675c31..94e226f637992 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -222,7 +222,9 @@ struct worker_pool {
 	struct workqueue_attrs	*attrs;		/* I: worker attributes */
 	struct hlist_node	hash_node;	/* PL: unbound_pool_hash node */
 	int			refcnt;		/* PL: refcnt for unbound pools */
-
+#ifdef CONFIG_PREEMPT_RT
+	spinlock_t		cb_lock;	/* BH worker cancel lock */
+#endif
 	/*
 	 * Destruction of pool is RCU protected to allow dereferences
 	 * from get_work_pool().
@@ -3078,6 +3080,31 @@ __acquires(&pool->lock)
 		goto restart;
 }
 
+#ifdef CONFIG_PREEMPT_RT
+static void worker_lock_callback(struct worker_pool *pool)
+{
+	spin_lock(&pool->cb_lock);
+}
+
+static void worker_unlock_callback(struct worker_pool *pool)
+{
+	spin_unlock(&pool->cb_lock);
+}
+
+static void workqueue_callback_cancel_wait_running(struct worker_pool *pool)
+{
+	spin_lock(&pool->cb_lock);
+	spin_unlock(&pool->cb_lock);
+}
+
+#else
+
+static void worker_lock_callback(struct worker_pool *pool) { }
+static void worker_unlock_callback(struct worker_pool *pool) { }
+static void workqueue_callback_cancel_wait_running(struct worker_pool *pool) { }
+
+#endif
+
 /**
  * manage_workers - manage worker pool
  * @worker: self
@@ -3557,6 +3584,7 @@ static void bh_worker(struct worker *worker)
 	int nr_restarts = BH_WORKER_RESTARTS;
 	unsigned long end = jiffies + BH_WORKER_JIFFIES;
 
+	worker_lock_callback(pool);
 	raw_spin_lock_irq(&pool->lock);
 	worker_leave_idle(worker);
 
@@ -3585,6 +3613,7 @@ static void bh_worker(struct worker *worker)
 	worker_enter_idle(worker);
 	kick_pool(pool);
 	raw_spin_unlock_irq(&pool->lock);
+	worker_unlock_callback(pool);
 }
 
 /*
@@ -4222,17 +4251,18 @@ static bool __flush_work(struct work_struct *work, bool from_cancel)
 		    (data & WORK_OFFQ_BH)) {
 			/*
 			 * On RT, prevent a live lock when %current preempted
-			 * soft interrupt processing or prevents ksoftirqd from
-			 * running by keeping flipping BH. If the BH work item
-			 * runs on a different CPU then this has no effect other
-			 * than doing the BH disable/enable dance for nothing.
-			 * This is copied from
-			 * kernel/softirq.c::tasklet_unlock_spin_wait().
+			 * soft interrupt processing by blocking on lock which
+			 * is owned by the thread invoking the callback.
 			 */
 			while (!try_wait_for_completion(&barr.done)) {
 				if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
-					local_bh_disable();
-					local_bh_enable();
+					struct worker_pool *pool;
+
+					mutex_lock(&wq_pool_mutex);
+					pool = get_work_pool(work);
+					if (pool)
+						workqueue_callback_cancel_wait_running(pool);
+					mutex_unlock(&wq_pool_mutex);
 				} else {
 					cpu_relax();
 				}
@@ -4782,6 +4812,9 @@ static int init_worker_pool(struct worker_pool *pool)
 	ida_init(&pool->worker_ida);
 	INIT_HLIST_NODE(&pool->hash_node);
 	pool->refcnt = 1;
+#ifdef CONFIG_PREEMPT_RT
+	spin_lock_init(&pool->cb_lock);
+#endif
 
 	/* shouldn't fail above this point */
 	pool->attrs = alloc_workqueue_attrs();
-- 
2.50.1