Re: [PATCH v2 REPOST] fscache: Use only one fscache_object_cong_wait.

[Sebastian Andrzej Siewior]

On 2021-11-18 17:54:44 [+0100], To linux-cachefs@xxxxxxxxxx wrote:
> 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 <gregor.beck@xxxxxxxxx>
> Cc: stable-rt@xxxxxxxxxxxxxxx
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>

I noticed that -next gained commit
   608bfec640edb ("fscache: Remove the contents of the fscache driver, pending rewrite")

which removes slow_work_sleep_till_thread_needed() and the per-CPU
variable. Since it looks like a bug, what happens stable wise?

> ---
> Previous post
>   https://lore.kernel.org/all/20211029083839.xwwt7jgzru3kcpii@xxxxxxxxxxxxx/
> 
>  fs/fscache/internal.h |    1 -
>  fs/fscache/main.c     |    6 ------
>  fs/fscache/object.c   |   13 +++++--------
>  3 files changed, 5 insertions(+), 15 deletions(-)
> 
> --- a/fs/fscache/internal.h
> +++ b/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);
>  
> --- a/fs/fscache/main.c
> +++ b/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;
> --- a/fs/fscache/object.c
> +++ b/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();
>  }

Sebastian