On 2021-12-02 21:52:45 [+0100], To linux-cachefs@xxxxxxxxxx wrote:
> 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?
a gentle ping.
> > ---
> > 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
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