From: Tang Junhui <tang.junhui@xxxxxxxxxx>
Hello Coly:
There are some differences,
Using variable of atomic_t type can not guarantee the atomicity of transaction.
for example:
A thread runs in update_writeback_rate()
update_writeback_rate(){
....
+ if (test_bit(BCACHE_DEV_WB_RUNNING, &dc->disk.flags)) {
+ schedule_delayed_work(&dc->writeback_rate_update,
dc->writeback_rate_update_seconds * HZ);
+ }
Then another thread executes in cached_dev_detach_finish():
if (test_and_clear_bit(BCACHE_DEV_WB_RUNNING, &dc->disk.flags))
cancel_writeback_rate_update_dwork(dc);
+
+ /*
+ * should check BCACHE_DEV_RATE_DW_RUNNING before calling
+ * cancel_delayed_work_sync().
+ */
+ clear_bit(BCACHE_DEV_RATE_DW_RUNNING, &dc->disk.flags);
+ /* paired with where BCACHE_DEV_RATE_DW_RUNNING is tested */
+ smp_mb();
Race still exists.
>
> On 29/01/2018 3:35 PM, tang.junhui@xxxxxxxxxx wrote:
> > From: Tang Junhui <tang.junhui@xxxxxxxxxx>
> >
> > Hello Coly:
> >
> > This patch is somewhat difficult for me,
> > I think we can resolve it in a simple way.
> >
> > We can take the schedule_delayed_work() under the protection of
> > dc->writeback_lock, and judge if we need re-arm this work to queue.
> >
> > static void update_writeback_rate(struct work_struct *work)
> > {
> > struct cached_dev *dc = container_of(to_delayed_work(work),
> > struct cached_dev,
> > writeback_rate_update);
> >
> > down_read(&dc->writeback_lock);
> >
> > if (atomic_read(&dc->has_dirty) &&
> > dc->writeback_percent)
> > __update_writeback_rate(dc);
> >
> > - up_read(&dc->writeback_lock);
> > + if (NEED_RE-AEMING)
> > schedule_delayed_work(&dc->writeback_rate_update,
> > dc->writeback_rate_update_seconds * HZ);
> > + up_read(&dc->writeback_lock);
> > }
> >
> > In cached_dev_detach_finish() and cached_dev_free() we can set the no need
> > flag under the protection of dc->writeback_lock, for example:
> >
> > static void cached_dev_detach_finish(struct work_struct *w)
> > {
> > ...
> > + down_write(&dc->writeback_lock);
> > + SET NO NEED RE-ARM FLAG
> > + up_write(&dc->writeback_lock);
> > cancel_delayed_work_sync(&dc->writeback_rate_update);
> > }
> >
> > I think this way is more simple and readable.
> >
>
> Hi Junhui,
>
> Your suggest is essentially almost same to my patch,
> - clear BCACHE_DEV_DETACHING bit acts as SET NO NEED RE-ARM FLAG.
> - cancel_writeback_rate_update_dwork acts as some kind of locking with a
> timeout.
>
> The difference is I don't use dc->writeback_lock, and replace it by
> BCACHE_DEV_RATE_DW_RUNNING.
>
> The reason is my following development. I plan to implement a real-time
> update stripe_sectors_dirty of bcache device and cache set, then
> bcache_flash_devs_sectors_dirty() can be very fast and bch_register_lock
> can be removed here. And then I also plan to remove reference of
> dc->writeback_lock in update_writeback_rate() because indeed it is
> unnecessary here (the patch is held by Mike's locking resort work).
>
> Since I plan to remove dc->writeback_lock from update_writeback_rate(),
> I don't want to reference dc->writeback in the delayed work.
>
> The basic idea behind your suggestion and this patch, is almost
> identical. The only difference might be the timeout in
> cancel_writeback_rate_update_dwork().
>
> Thanks.
>
> Coly Li
Thanks.
Tang Junhui
