Hi! On Thu 11-06-20 10:34:17, Lukas Czerner wrote: > I am tracking a rare and very hard to reproduce bug that ends up hittng > > J_ASSERT_JH(jh, jh->b_transaction == NULL) > > in __journal_remove_journal_head(). In fact we can get there with > b_next_transaction set and b_jlist == BJ_Forget so it's clear that we > should not have dropped the last JH reference at that point. > > Most of the time that I've seen we get there from > __jbd2_journal_remove_checkpoint() called from > jbd2_journal_commit_transaction(). > > The locking in and around grabbing and putting the journal head > reference (b_jcount) looks solid as well as the use of j_list_lock. But > I have noticed a problem in logic of __jbd2_journal_refile_buffer(). Yeah, the trouble with refcounting bugs is that if *any* of the users releases a reference it should not, we will (much later) hit the problem you describe. > The idea is that b_next_transaction will inherit the reference from > b_transaction so that we do not need to grab a new reference of > journal_head. However this will only be true if b_transaction is set. > > But if it is indeed NULL, then we will do > > WRITE_ONCE(jh->b_transaction, jh->b_next_transaction); > > and __jbd2_journal_file_buffer() will not grab the jh reference. AFAICT > the b_next_transaction is not holding it's own jh reference. This will > result in b_transaction _not_ holding it's own jh reference and we will > be able to drop the last jh reference at unexpected places - hence we can > hit the asserts in __journal_remove_journal_head(). > > However I am not really sure if it is indeed possible to get into > __jbd2_journal_refile_buffer() with b_transaction == NULL and > b_next_transaction set. Jan do you have any idea if that's possible and > what would be the circumstances to lead us there ? __jbd2_journal_refile_buffer() should always be called with b_transaction != NULL and as I've checked (all three) callers, that indeed seems to be the case. Feel free to add assert along those lines to __jbd2_journal_refile_buffer() to see whether it triggers... > Regardless I still think this is a bug in the logic and we should either > make sure that b_transaction is _not_ NULL in > __jbd2_journal_refile_buffer(), or let __jbd2_journal_file_buffer() grab > the jh reference if b_transaction was indeen NULL. How about something > like the following untested patch ? I'd rather got for the assert. It makes things simpler, also the "meaning" of __jbd2_journal_refile_buffer() is "jh is done in its current transaction, deal with it" and that doesn't have a great meaning if b_transaction is NULL. And when you're adding asserts, then adding one in __jbd2_journal_unfile_buffer() checking b_transaction != NULL and b_next_transaction == NULL would be good as well because lot of callers assume this. I've checked the code and I didn't find any problematic one but that code is complex enough that I could have missed something. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
