On 01/03/2018 03:03 PM, Coly Li wrote: > struct delayed_work writeback_rate_update in struct cache_dev is a delayed > worker to call function update_writeback_rate() in period (the interval is > defined by dc->writeback_rate_update_seconds). > > When a metadate I/O error happens on cache device, bcache error handling > routine bch_cache_set_error() will call bch_cache_set_unregister() to > retire whole cache set. On the unregister code path, cached_dev_free() > calls cancel_delayed_work_sync(&dc->writeback_rate_update) to stop this > delayed work. > > dc->writeback_rate_update is a special delayed work from others in bcache. > In its routine update_writeback_rate(), this delayed work is re-armed > after a piece of time. That means when cancel_delayed_work_sync() returns, > this delayed work can still be executed after several seconds defined by > dc->writeback_rate_update_seconds. > > The problem is, after cancel_delayed_work_sync() returns, the cache set > unregister code path will eventually release memory of struct cache set. > Then the delayed work is scheduled to run, and inside its routine > update_writeback_rate() that already released cache set NULL pointer will > be accessed. Now a NULL pointer deference panic is triggered. > > In order to avoid the above problem, this patch checks cache set flags in > delayed work routine update_writeback_rate(). If flag CACHE_SET_STOPPING > is set, this routine will quit without re-arm the delayed work. Then the > NULL pointer deference panic won't happen after cache set is released. > > Signed-off-by: Coly Li <colyli@xxxxxxx> > --- > drivers/md/bcache/writeback.c | 9 +++++++++ > 1 file changed, 9 insertions(+) > > diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c > index 0789a9e18337..745d9b2a326f 100644 > --- a/drivers/md/bcache/writeback.c > +++ b/drivers/md/bcache/writeback.c > @@ -91,6 +91,11 @@ 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); > + struct cache_set *c = dc->disk.c; > + > + /* quit directly if cache set is stopping */ > + if (test_bit(CACHE_SET_STOPPING, &c->flags)) > + return; > > down_read(&dc->writeback_lock); > > @@ -100,6 +105,10 @@ static void update_writeback_rate(struct work_struct *work) > > up_read(&dc->writeback_lock); > > + /* do not schedule delayed work if cache set is stopping */ > + if (test_bit(CACHE_SET_STOPPING, &c->flags)) > + return; > + > schedule_delayed_work(&dc->writeback_rate_update, > dc->writeback_rate_update_seconds * HZ); > } > This is actually not quite correct; the function might still be called after 'struct cached_dev' has been removed. The correct way of fixing is to either take a reference to struct cached_dev and release it once 'update_writeback_rate' is finished, or to call 'cancel_delayed_work_sync()' before deleting struct cached_dev. Cheers, Hannes -- Dr. Hannes Reinecke Teamlead Storage & Networking hare@xxxxxxx +49 911 74053 688 SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: F. Imendörffer, J. Smithard, J. Guild, D. Upmanyu, G. Norton HRB 21284 (AG Nürnberg)
