On 04/01/2018 1:27 AM, tang.junhui@xxxxxxxxxx wrote: > From: Tang Junhui <tang.junhui@xxxxxxxxxx> > > Hello Coly, > Thanks for your works! > Hi Junhui, > Acctually stopping write-back thread and writeback_rate_update work in > bcache_device_detach() has already done in: > https://github.com/mlyle/linux/commit/397d02e162b8ee11940a4e9f45e16fee0650d64e >> Is it nessary to add "rate_update_canceled" to identify that > whether writeback_rate_update work are already stoped or not? > I think it is ok to call cancel_delayed_work_sync(&dc->writeback_rate_update) > twice before the memory of writeback_rate_update being released (It will return > -ENOENT). > Good suggestion, I just find after cancel_delayed_work_sync() returns the worker is in idle state, so call cancel_delayed_work_sync() to an idle worker will return 0 immediately, so rate_update_canceled is unnecessary. It seems your patch is enough and does the job. Let me test v2 patch set against bcache-for-next, if it works I will remove this patch from my patch set. Thanks for your hint :-) Coly Li >> Delayed worker dc->writeback_rate_update and kernel thread >> dc->writeback_thread reference cache set data structure in their routine, >> Therefor, before they are stopped, cache set should not be release. Other- >> wise, NULL pointer deference will be triggered. >> >> Currenly delayed worker dc->writeback_rate_update and kernel thread >> dc->writeback_thread are stopped in cached_dev_free(). When cache set is >> retiring by too many I/O errors, cached_dev_free() is called when refcount >> of bcache device's closure (disk.cl) reaches 0. In most of cases, last >> refcount of disk.cl is dropped in last line of cached_dev_detach_finish(). >> But in cached_dev_detach_finish() before calling closure_put(&dc->disk.cl), >> bcache_device_detach() is called, and inside bcache_device_detach() >> refcount of cache_set->caching is dropped by closure_put(&d->c->caching). >> >> It is very probably this is the last refcount of this closure, so routine >> cache_set_flush() will be called (it is set in __cache_set_unregister()), >> and its parent closure cache_set->cl may also drop its last refcount and >> cache_set_free() is called too. In cache_set_free() the last refcount of >> cache_set->kobj is dropped and then bch_cache_set_release() is called. Now >> in bch_cache_set_release(), the memory of struct cache_set is freeed. >> >> bch_cache_set_release() is called before cached_dev_free(), then there is a >> time window after cache set memory freed and before dc->writeback_thread >> and dc->writeback_rate_update stopped, if one of them is scheduled to run, >> a NULL pointer deference will be triggered. >> >> This patch fixes the above problem by stopping dc->writeback_thread and >> dc->writeback_rate_update earlier in bcache_device_detach() before calling >> closure_put(&d->c->caching). Because cancel_delayed_work_sync() and >> kthread_stop() are synchronized operations, we can make sure cache set >> is available when the delayed work and kthread are stopping. >> >> Because cached_dev_free() can also be called by writing 1 to sysfs file >> /sys/block/bcache<N>/bcache/stop, this code path may not call >> bcache_device_detach() if d-c is NULL. So stopping dc->writeback_thread >> and dc->writeback_rate_update in cached_dev_free() is still necessary. In >> order to avoid stop them twice, dc->rate_update_canceled is added to >> indicate dc->writeback_rate_update is canceled, and dc->writeback_thread >> is set to NULL to indicate it is stopped. >> >> Signed-off-by: Coly Li <colyli@xxxxxxx> [code snip] -- To unsubscribe from this list: send the line "unsubscribe linux-bcache" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
