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>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
v2: https://lore.kernel.org/all/20211029083839.xwwt7jgzru3kcpii@xxxxxxxxxxxxx/
Repost: https://lore.kernel.org/all/20211118165442.hekmz7xgisdzsyuh@xxxxxxxxxxxxx/
Ping 1: https://lore.kernel.org/all/20211202205240.giqxuxqemlxxoobw@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?
Ping 2: https://lore.kernel.org/all/YbdiYN+wU1RN9mWo@xxxxxxxxxxxxx/
fs/fscache/internal.h | 1 -
fs/fscache/main.c | 6 ------
fs/fscache/object.c | 13 +++++--------
3 files changed, 5 insertions(+), 15 deletions(-)
diff --git a/fs/fscache/internal.h b/fs/fscache/internal.h
index c3e4804b8fcbf..9edb87e11680b 100644
--- 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);
diff --git a/fs/fscache/main.c b/fs/fscache/main.c
index 4207f98e405fd..85f8cf3a323d5 100644
--- 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 salt, unsigned int *data, unsigned int n)
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;
diff --git a/fs/fscache/object.c b/fs/fscache/object.c
index 6a675652129b2..7a972d144b546 100644
--- a/fs/fscache/object.c
+++ b/fs/fscache/object.c
@@ -798,6 +798,8 @@ void fscache_object_destroy(struct fscache_object *object)
}
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 fscache_object *object)
_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 fscache_object *object)
*/
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();
}
--
2.34.1
