Re: [PATCH v6 06/12] monitor: Make current monitor a per-coroutine property

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Aug 04, 2020 at 03:50:54PM +0200, Markus Armbruster wrote:
> Kevin Wolf <kwolf@redhat.com> writes:
> 
> > This way, a monitor command handler will still be able to access the
> > current monitor, but when it yields, all other code code will correctly
> > get NULL from monitor_cur().
> >
> > Outside of coroutine context, qemu_coroutine_self() returns the leader
> > coroutine of the current thread.
> 
> Unsaid: you use it as a hash table key to map from coroutine to monitor,
> and for that you need it to return a value unique to the coroutine in
> coroutine context, and a value unique to the thread outside coroutine
> context.  Which qemu_coroutine_self() does.  Correct?
> 
> The hash table works, but I hate it just as much as I hate
> pthread_getspecific() / pthread_setspecific().
> 
> What we have here is a need for coroutine-local data.  Feels like a
> perfectly natural concept to me.
> 
> Are we going to create another hash table whenever we need another piece
> of coroutine-local data?  Or shall we reuse the hash table, suitably
> renamed and moved to another file?
> 
> Why not simply associate an opaque pointer with each coroutine?  All it
> takes is one more member of struct Coroutine.  Whatever creates the
> coroutine decides what to use it for.  The monitor coroutine would use
> it to point to the monitor.

Possible benefit of having the coroutine-local data stored in the
coroutine stack is that we can probably make it lock-less. Using
the hash table in monitor.c results in a serialization of across
all coroutines & threads.

Also, by providing a GDestroyNotify against the coroutine-local
data we can easily guarantee cleanup with the coroutine is freed.

Since we'll have a limited number of data items, we could make do
with a simple array in the coroutine struct, instead of a hashtable.
eg

  enum CoroutineLocalKeys {
     CO_LOCAL_CUR_MONITOR = 0,

     CO_LOCAL_LAST,
  };

  struct Coroutine {
    ...
    gpointer localData[CO_LOCAL_LAST];
    GDestroyNotify localDataFree[CO_LOCAL_LAST];
  };


Regards,
Daniel
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