From: Tang Junhui <tang.junhui@xxxxxxxxxx> Hi Coly, >It is about an implicit and interesting ordering, a simple patch with a >lot detail behind. Let me explain why it's safe, > >- cancel_delayed_work_sync() is called in bcache_device_detach() when >dc->count is 0. But cancel_delayed_work_sync() is called before >closure_put(&d->c->caching) in bcache_device_detach(). > >- After closure_put(&d->c->caching) called in bcache_device_detach(), >refcount of c->caching becomes 0, and cache_set_flush() set in >__cache_set_unregister() continue to execute. > >- See continue_at() and closure_put_after_sub(), after c->caching >refoucnt reaches 0, and c->caching->fn gets called, closure_put(parent) >will be called. Here the parent of c->caching is c->cl, so refcount of >c->cl will reach 0, and closure_put_after_sub() will be called again for >c->cl and call its cl->fn which is cache_set_free(). It is set in >bch_cache_set_alloc(). > >- So before closure_put(&d->c->caching) is called in >bcache_device_detach(), cache_set_flush() in __cache_set_unregister() >won't be executed and memory of cache_set won't be freed. > >Now back to delayed worker routine update_writeback_rate(), I check >c->flags inside it, in this moment cancel_delayed_work_sync() is still >waiting for update_writeback_rate() to exit, so refcount of c->caching >is still hold and cache set memory will only be freed after >cancel_delayed_work_sync() returns and refcount of c->caching dropped. > >And in cached_dev_free(), bcache_device_free() and >kobject_put(&dc->disk.kobj) are called after cancel_delayed_work_sync() >returns. Therefore when update_writeback_rate() is executing and >accessing c->flgas inside, memory of cache_set, bcache_device and >cached_dev are all there. OK, I got your mean. It is safe run work update_writeback_rate when cancel_delayed_work_sync() is called, but it is not safe to re-arm itself to workqueue again. So, the first judgement "if (test_bit(CACHE_SET_STOPPING, &c->flags))" is not very nessary, we only need the second one to prevent the work from re-arming itself to workqueue. Thanks, Tang Junhui
