Re: [Xen-devel] /proc/xen/xenbus supports watch?

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 25 Sep 2005, at 01:57, Rusty Russell wrote:

> Now, we already have this "domain won't save until transactions are
> done" simply because we use a single big lock, but this discussion
> started because we want to get rid of that lock for /proc/xen/xenbus
> (it's fine for drivers).  I think we should do so, but keep this
> wont-save-during-transactions semantic; it means a waitqueue etc, but I
> don't think it's too bad.  As you say, our transactions are pretty
> small.
>
> Do people like this more?
Blocking system progress on arbitrary user apps doesn't sound 
particularly attractive to me, but I guess it is at least simple. I'm 
more inclined to EAGAIN on read/write, maybe sugared by exceptions in 
languages that support them, or setjmp/longjmp to get you back to the 
outermost scope of the transaction. I agree that's not pretty either, 
though.
How will you handle 'xenstored restart'? You can't really guarantee 
that to always happen at opportune moments with no transactions in 
flight.
 -- Keir


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