On Tue, Aug 22, 2017 at 02:33:48PM +0800, Fam Zheng wrote: > On Tue, 08/22 13:59, Peter Xu wrote: > > On Tue, Aug 22, 2017 at 12:15:19PM +0800, Fam Zheng wrote: > > > On Tue, 08/22 10:56, Peter Xu wrote: > > > > I haven't really encountered (c), but I think it's the migrate_cancel > > > > command that matters, which should not need BQL as well. > > > > > > There is bdrv_invalidate_cache_all() in migrate_cancel which clearly > > > isn't safe. > > > Is that if block unreachable in this case? If so we should assert, > > > otherwise > > > this command is not okay to run without BQL. > > > > Ah. I see. Even if so, if that is the only usage of BQL, IMHO we can > > still mark migrate_cancel as "without-bql=true", instead we take the > > BQL before calling bdrv_invalidate_cache_all(). Then migrate_cancel > > can be BQL-free at least when block migration is not active. > > > > > > > > Generically, what guarantee the thread-safety of a qmp command when you > > > decide > > > BQL is not needed? In other words, how do you prove commands are safe > > > without > > > BQL? I think almost every command accesses global state, but lock-free > > > data > > > structures are rare AFAICT. > > > > I would suggest we split the problem into at least three parts. IMHO > > we need to answer below questions one by one to know what we should do > > next: > > > > 1. whether we can handle monitor commands outside iothread, or say, in > > an isolated thread? > > > > This is basically what patch 2 does, the "per-monitor threads". > > > > IMHO this is the very first question to ask. So now I know that at > > least current code cannot do it. We need to at least do something > > to remove/replace the assertion to make this happen. Can we? I > > don't really know the answer yet. If this is undoable, we can skip > > question 2/3 below and may need to rethink on how to solve the > > problem that postcopy recovery encounters. > > > > 2. whether there is any monitor commands can run without BQL? > > > > This is basically what patch 3/5 does, one for QMP, one for HMP. > > > > If we can settle question 1, then we can possibly start consider > > this question. This step does not really allow any command to run > > without BQL, but we need to know whether it's possible in general, > > and if possible, we provide a framework to allow QMP/HMP developers > > to specify that. If you see patch 3/5, the default behavior is > > still taking the BQL for all commands. > > > > IMHO doing this whole thing is generally good in the sense that > > this is actually forcing ourselves to break the BQL into smaller > > locks. Take the migration commands for example: migrate_incoming > > do not need BQL, and when we write codes around it we know that we > > don't need to think about thread-safety. That's not good IMHO. I > > think it's time we should start consider thread-safety always. > > Again, for migrate_incoming to do this, actually we'll possibly at > > least need a migration management lock (the smaller lock) to make > > sure e.g. the user is not running two migrate_incoming commands in > > parallel (after per-monitor threads, it can happen). But it's > > better than BQL, because BQL is for sure too big, so even a guest > > page access (as long as it held the BQL) can block migration > > commands. > > Yes, this is my point. You cannot just declare a command "BQL-free" without > adding small locks first, and I think this is actually missing in this series. > As you said, two per-monitor threads can race if they do migrate_incoming in > parallel. This is also the answer to 3.
Ah, I see. The small lock will be there if there is another post. :) -- Peter Xu