[PATCH v2] rhashtable: Bounce deferred worker kick through irq_work

[Tejun Heo]

Inserts past 75% load call schedule_work(&ht->run_work) to kick an
async resize. If a caller holds a raw spinlock (e.g. an
insecure_elasticity user), schedule_work() under that lock records

  caller_lock -> pool->lock -> pi_lock -> rq->__lock

A cycle forms if any of these locks is acquired in the reverse
direction elsewhere. sched_ext, the only current insecure_elasticity
user, hits this: it holds scx_sched_lock across rhashtable inserts of
sub-schedulers, while scx_bypass() takes rq->__lock -> scx_sched_lock.
Exercising the resize path produces:

  Chain exists of:
    &pool->lock --> &rq->__lock --> scx_sched_lock

Route the kick unconditionally through irq_work so schedule_work() runs
from hard IRQ context with the caller's lock no longer held.

v2: bounce unconditionally instead of gating on insecure_elasticity, as
    suggested by Herbert.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
---
Herbert, any preference on how this should be routed?

Thanks.

 include/linux/rhashtable-types.h |  3 +++
 include/linux/rhashtable.h       |  3 ++-
 lib/rhashtable.c                 | 24 ++++++++++++++++++++----
 3 files changed, 25 insertions(+), 5 deletions(-)

diff --git a/include/linux/rhashtable-types.h b/include/linux/rhashtable-types.h
index 72082428d6c6..fc2f596a6df1 100644
--- a/include/linux/rhashtable-types.h
+++ b/include/linux/rhashtable-types.h
@@ -12,6 +12,7 @@
 #include <linux/alloc_tag.h>
 #include <linux/atomic.h>
 #include <linux/compiler.h>
+#include <linux/irq_work_types.h>
 #include <linux/mutex.h>
 #include <linux/workqueue_types.h>

@@ -77,6 +78,7 @@ struct rhashtable_params {
  * @p: Configuration parameters
  * @rhlist: True if this is an rhltable
  * @run_work: Deferred worker to expand/shrink asynchronously
+ * @run_irq_work: Bounces the @run_work kick through hard IRQ context.
  * @mutex: Mutex to protect current/future table swapping
  * @lock: Spin lock to protect walker list
  * @nelems: Number of elements in table
@@ -88,6 +90,7 @@ struct rhashtable_params {
 	struct rhashtable_params	p;
 	bool				rhlist;
 	struct work_struct		run_work;
+	struct irq_work			run_irq_work;
 	struct mutex                    mutex;
 	spinlock_t			lock;
 	atomic_t			nelems;
diff --git a/include/linux/rhashtable.h b/include/linux/rhashtable.h
index 7def3f0f556b..ef5230cece36 100644
--- a/include/linux/rhashtable.h
+++ b/include/linux/rhashtable.h
@@ -20,6 +20,7 @@

 #include <linux/err.h>
 #include <linux/errno.h>
+#include <linux/irq_work.h>
 #include <linux/jhash.h>
 #include <linux/list_nulls.h>
 #include <linux/workqueue.h>
@@ -847,7 +848,7 @@ static __always_inline void *__rhashtable_insert_fast(
 	rht_assign_unlock(tbl, bkt, obj, flags);

 	if (rht_grow_above_75(ht, tbl))
-		schedule_work(&ht->run_work);
+		irq_work_queue(&ht->run_irq_work);

 	data = NULL;
 out:
diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index fb2b7bc137ba..218d3c1f34fb 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -442,7 +442,21 @@ static void rht_deferred_worker(struct work_struct *work)
 	mutex_unlock(&ht->mutex);

 	if (err)
-		schedule_work(&ht->run_work);
+		irq_work_queue(&ht->run_irq_work);
+}
+
+/*
+ * rhashtable can be used under raw spinlocks. Calling schedule_work()
+ * from such context can close a locking cycle through workqueue and
+ * scheduler locks. Bounce through irq_work so the schedule_work() runs
+ * from hard IRQ context with the caller's lock no longer held.
+ */
+static void rht_deferred_irq_work(struct irq_work *irq_work)
+{
+	struct rhashtable *ht = container_of(irq_work, struct rhashtable,
+					     run_irq_work);
+
+	schedule_work(&ht->run_work);
 }

 static int rhashtable_insert_rehash(struct rhashtable *ht,
@@ -477,7 +491,7 @@ static int rhashtable_insert_rehash(struct rhashtable *ht,
 		if (err == -EEXIST)
 			err = 0;
 	} else
-		schedule_work(&ht->run_work);
+		irq_work_queue(&ht->run_irq_work);

 	return err;

@@ -488,7 +502,7 @@ static int rhashtable_insert_rehash(struct rhashtable *ht,

 	/* Schedule async rehash to retry allocation in process context. */
 	if (err == -ENOMEM)
-		schedule_work(&ht->run_work);
+		irq_work_queue(&ht->run_irq_work);

 	return err;
 }
@@ -630,7 +644,7 @@ static void *rhashtable_try_insert(struct rhashtable *ht, const void *key,
 			rht_unlock(tbl, bkt, flags);

 			if (inserted && rht_grow_above_75(ht, tbl))
-				schedule_work(&ht->run_work);
+				irq_work_queue(&ht->run_irq_work);
 		}
 	} while (!IS_ERR_OR_NULL(new_tbl));

@@ -1085,6 +1099,7 @@ int rhashtable_init_noprof(struct rhashtable *ht,
 	RCU_INIT_POINTER(ht->tbl, tbl);

 	INIT_WORK(&ht->run_work, rht_deferred_worker);
+	init_irq_work(&ht->run_irq_work, rht_deferred_irq_work);

 	return 0;
 }
@@ -1150,6 +1165,7 @@ void rhashtable_free_and_destroy(struct rhashtable *ht,
 	struct bucket_table *tbl, *next_tbl;
 	unsigned int i;

+	irq_work_sync(&ht->run_irq_work);
 	cancel_work_sync(&ht->run_work);

 	mutex_lock(&ht->mutex);