From fecf013191e3a5b98b7a800074114a5b40ceafaa Mon Sep 17 00:00:00 2001 From: Sebastian Andrzej Siewior Date: Thu, 28 Oct 2021 17:30:50 +0200 Subject: [PATCH 059/158] 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 Cc: stable-rt@vger.kernel.org Signed-off-by: Sebastian Andrzej Siewior Link: https://lkml.kernel.org/r/20211029083839.xwwt7jgzru3kcpii@linutronix.de --- fs/fscache/internal.h | 1 - fs/fscache/main.c | 6 ------ fs/fscache/object.c | 13 +++++-------- 3 files changed, 5 insertions(+), 15 deletions(-) Index: linux-5.15.19-rt29/fs/fscache/internal.h =================================================================== --- linux-5.15.19-rt29.orig/fs/fscache/internal.h +++ linux-5.15.19-rt29/fs/fscache/internal.h @@ -81,7 +81,6 @@ 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); Index: linux-5.15.19-rt29/fs/fscache/main.c =================================================================== --- linux-5.15.19-rt29.orig/fs/fscache/main.c +++ linux-5.15.19-rt29/fs/fscache/main.c @@ -41,8 +41,6 @@ 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; @@ -138,7 +136,6 @@ 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 = @@ -161,9 +158,6 @@ 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; Index: linux-5.15.19-rt29/fs/fscache/object.c =================================================================== --- linux-5.15.19-rt29.orig/fs/fscache/object.c +++ linux-5.15.19-rt29/fs/fscache/object.c @@ -798,6 +798,8 @@ void fscache_object_destroy(struct fscac } EXPORT_SYMBOL(fscache_object_destroy); +static DECLARE_WAIT_QUEUE_HEAD(fscache_object_cong_wait); + /* * enqueue an object for metadata-type processing */ @@ -806,16 +808,12 @@ 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); } } @@ -833,16 +831,15 @@ 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); DEFINE_WAIT(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(); }