Re: [PATCH v2 6/8] kvm tools: Add rwlock wrapper

[Ingo Molnar <mingo@elte.hu>]

* Sasha Levin <levinsasha928@gmail.com> wrote:

> On Mon, 2011-05-30 at 12:13 +0200, Ingo Molnar wrote:
> > * Sasha Levin <levinsasha928@gmail.com> wrote:
> > 
> > > On Mon, 2011-05-30 at 11:56 +0200, Ingo Molnar wrote:
> > > > * Sasha Levin <levinsasha928@gmail.com> 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