rhashtable: Bounce deferred worker kick through irq_work

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

Bounce the kick from the insert paths through irq_work so
schedule_work() runs from hard IRQ context with the caller's lock no
longer held. rht_deferred_worker()'s self-rearm on error stays on
schedule_work(&ht->run_work) - the worker runs in process context with
no caller lock held, and keeping the self-requeue on @run_work lets
cancel_work_sync() in rhashtable_free_and_destroy() drain it.

v3: Keep rht_deferred_worker()'s self-rearm on schedule_work(&run_work).
    Routing it through irq_work in v2 broke cancel_work_sync()'s
    self-requeue handling - an irq_work queued after irq_work_sync()
    returned but while cancel_work_sync() was still waiting could fire
    post-teardown.

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

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>

1 files changed, 3 insertions, 0 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 {
 	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;