From: Sebastian Andrzej Siewior <>
Date: Thu, 28 Oct 2021 17:30:50 +0200
Subject: [PATCH] fscache: Use only one fscache_object_cong_wait.

In the commit mentioned below, fscache was converted from slow-work to
workqueue. slow_work_enqueue() and slow_work_sleep_till_thread_needed()
did not use a per-CPU workqueue. They choose from two global waitqueues
depending on the SLOW_WORK_VERY_SLOW bit which was not set so it always
one waitqueue.

I can't find out how it is ensured that a waiter on certain CPU is woken
up be the other side. My guess is that the timeout in schedule_timeout()
ensures that it does not wait forever (or a random wake up).

fscache_object_sleep_till_congested() must be invoked from preemptible
context in order for schedule() to work. In this case this_cpu_ptr()
should complain with CONFIG_DEBUG_PREEMPT enabled except the thread is
bound to one CPU.

wake_up() wakes only one waiter and I'm not sure if it is guaranteed
that only one waiter exists.

Replace the per-CPU waitqueue with one global waitqueue.

Fixes: 8b8edefa2fffb ("fscache: convert object to use workqueue instead of slow-work")
Reported-by: Gregor Beck <>
Signed-off-by: Sebastian Andrzej Siewior <>
 fs/fscache/internal.h |    1 -
 fs/fscache/main.c     |    6 ------
 fs/fscache/object.c   |   13 +++++--------
 3 files changed, 5 insertions(+), 15 deletions(-)

@ fs/fscache/internal.h:84 @ extern unsigned fscache_debug;
 extern struct kobject *fscache_root;
 extern struct workqueue_struct *fscache_object_wq;
 extern struct workqueue_struct *fscache_op_wq;
-DECLARE_PER_CPU(wait_queue_head_t, fscache_object_cong_wait);
 extern unsigned int fscache_hash(unsigned int salt, unsigned int *data, unsigned int n);
--- a/fs/fscache/main.c
+++ b/fs/fscache/main.c
@ fs/fscache/internal.h:44 @ struct kobject *fscache_root;
 struct workqueue_struct *fscache_object_wq;
 struct workqueue_struct *fscache_op_wq;
-DEFINE_PER_CPU(wait_queue_head_t, fscache_object_cong_wait);
 /* these values serve as lower bounds, will be adjusted in fscache_init() */
 static unsigned fscache_object_max_active = 4;
 static unsigned fscache_op_max_active = 2;
@ fs/fscache/internal.h:139 @ unsigned int fscache_hash(unsigned int s
 static int __init fscache_init(void)
 	unsigned int nr_cpus = num_possible_cpus();
-	unsigned int cpu;
 	int ret;
 	fscache_object_max_active =
@ fs/fscache/internal.h:161 @ static int __init fscache_init(void)
 	if (!fscache_op_wq)
 		goto error_op_wq;
-	for_each_possible_cpu(cpu)
-		init_waitqueue_head(&per_cpu(fscache_object_cong_wait, cpu));
 	ret = fscache_proc_init();
 	if (ret < 0)
 		goto error_proc;
--- a/fs/fscache/object.c
+++ b/fs/fscache/object.c
@ fs/fscache/internal.h:801 @ void fscache_object_destroy(struct fscac
+static DECLARE_WAIT_QUEUE_HEAD(fscache_object_cong_wait);
  * enqueue an object for metadata-type processing
@ fs/fscache/internal.h:811 @ void fscache_enqueue_object(struct fscac
 	_enter("{OBJ%x}", object->debug_id);
 	if (fscache_get_object(object, fscache_obj_get_queue) >= 0) {
-		wait_queue_head_t *cong_wq =
-			&get_cpu_var(fscache_object_cong_wait);
 		if (queue_work(fscache_object_wq, &object->work)) {
 			if (fscache_object_congested())
-				wake_up(cong_wq);
+				wake_up(&fscache_object_cong_wait);
 		} else
 			fscache_put_object(object, fscache_obj_put_queue);
-		put_cpu_var(fscache_object_cong_wait);
@ fs/fscache/internal.h:834 @ void fscache_enqueue_object(struct fscac
 bool fscache_object_sleep_till_congested(signed long *timeoutp)
-	wait_queue_head_t *cong_wq = this_cpu_ptr(&fscache_object_cong_wait);
 	if (fscache_object_congested())
 		return true;
-	add_wait_queue_exclusive(cong_wq, &wait);
+	add_wait_queue_exclusive(&fscache_object_cong_wait, &wait);
 	if (!fscache_object_congested())
 		*timeoutp = schedule_timeout(*timeoutp);
-	finish_wait(cong_wq, &wait);
+	finish_wait(&fscache_object_cong_wait, &wait);
 	return fscache_object_congested();