Andriy, Thanks for the context. I see that in freebsd the zfsdev_state_lock is the spa_namespace_lock. I agree with your analysis and am curious why these two locks are the same. I wonder if there are other potential deadlocks that could exist in the zvol code as a result of this.
Thanks, George On Fri, Jul 8, 2016 at 10:09 AM, Andriy Gapon <a...@freebsd.org> wrote: > > George, > > thank you very much for the reply. > I've got curious about this while looking into what appears to be a > FreeBSD-specific deadlock: > https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=203864#c13 > > > On 08/07/2016 17:03, George Wilson wrote: > > Andriy, > > > > I think this is basically a rule, although I don't think it's stated > > anywhere. We do rely heavily on this locking strategy since there are > > many places that will hold the namespace lock to prevent spa config > > changes but yet wait for a txg to sync. > > > > Is it honored everywhere? Well, I hope so but there probably hasn't been > > a detailed analysis done to see if there might be places where we do > > something like a spa_open() from some obscure path in the sync thread. I > > would hope that our testing would have discovered that quickly and > > hopefully developers will sufficiently exercise their code to make sure > > they don't violate this. > > > > Have you seen instances where we deadlock as a result of this? > > > > Thanks, > > George > > > > On Fri, Jul 8, 2016 at 9:40 AM, Andriy Gapon <a...@freebsd.org > > <mailto:a...@freebsd.org>> wrote: > > > > > > I see a few places in the code that do the following: > > mutex_enter(&spa_namespace_lock); > > dsl_sync_task(...); > > mutex_exit(&spa_namespace_lock); > > One example is spa_change_guid(). > > > > In my understanding this implies that the sync thread must never > acquire > > spa_namespace_lock. Is this a real rule? Do we always honor it? > > > > Thank you! > > -- > > Andriy Gapon > > > > > > http://www.listbox.com > > > > > > *openzfs-developer* | Archives > > <https://www.listbox.com/member/archive/274414/=now> > > <https://www.listbox.com/member/archive/rss/274414/28133750-22ed9730> | > > Modify > > <https://www.listbox.com/member/?&;> > > Your Subscription [Powered by Listbox] <http://www.listbox.com> > > > > > -- > Andriy Gapon > ------------------------------------------- openzfs-developer Archives: https://www.listbox.com/member/archive/274414/=now RSS Feed: https://www.listbox.com/member/archive/rss/274414/28015062-cce53afa Modify Your Subscription: https://www.listbox.com/member/?member_id=28015062&id_secret=28015062-f966d51c Powered by Listbox: http://www.listbox.com
