* Sasha Levin <levinsasha928@xxxxxxxxx> wrote: > On Mon, 2011-05-30 at 12:13 +0200, Ingo Molnar wrote: > > * Sasha Levin <levinsasha928@xxxxxxxxx> wrote: > > > > > On Mon, 2011-05-30 at 11:56 +0200, Ingo Molnar wrote: > > > > * Sasha Levin <levinsasha928@xxxxxxxxx> wrote: > > > > > > > > > I'm just saying that we're limited to as many VCPU threads as we > > > > > can create. br_read_lock() won't do anything on a non-VCPU thread, > > > > > which makes it impossible to test it on non-VCPUs. > > > > > > > > btw., i wondered about that limit - don't we want to fix it? > > > > > > > > I mean, there's no fundamental reason why brlocks should do 'nothing' > > > > in worker threads. In fact it's a subtle breakage waiting AFAICS. > > > > > > Can they do anything useful without locking? I think we should work > > > on integrating an RCU and changing brlocks to use that instead of > > > focusing too much on the current implementation. > > > > What do you mean 'without locking'? If a worker thread uses a > > br_read_lock() then that will be 'locking'. It should map to a real > > read_lock() in the rwlock debug case, etc. > > > I meant without locking anything within br_read_lock(), because we > wanted to keep the read patch lock-free. oh, so it's not recursive. Sane enough - might be worth adding: br_is_read_locked(&lock) and a debug check for that into br_read_lock(): BUG_ON(br_is_read_locked(&lock)); > > > This will also fix that limit you don't like :) > > > > I'd prefer brlocks to more complex solutions in cases where the write > > path is very infrequent! > > > > So we don't want to keep brlocks intentionally crippled. > > Do you see brlock as a global lock that will pause the entire guest > (not just VCPUs - anything except the calling thread)? Yeah, that's how such brlocks work - life has to stop when there's write modifications going on. There should be a mutex around br_write_lock() itself, to make sure two br_write_lock() attempts cannot deadlock each other, but other than that it should be pretty straightforward and robust. And note that such a pause/suspend thing might be helpful to do a *real* host driven suspend feature in the future: stop all vcpus, all worker threads, save state to disk and exit? Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
