Re: [Xen-devel] [PATCH 23/24] libxl: child processes cleanups

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2012-04-24 at 15:27 +0100, Ian Jackson wrote:
> Ian Campbell writes ("Re: [Xen-devel] [PATCH 23/24] libxl: child processes 
> cleanups"):
> > On Mon, 2012-04-16 at 18:18 +0100, Ian Jackson wrote:
> > > Abolish libxl_fork.  Its only callers were in xl.  Its functionality
> > > is now moved elsewhere, as follows:
> ...
> > >  static inline int libxl__ev_child_inuse(libxl__ev_child *childw_out)
> > >                  { return childw_out->pid >= 0; }
> > >  
> > > +/* Useable (only) in the child to once more make the ctx useable for
> > > + * xenstore operations.
> > 
> > Specifically "the child" is the middle child of a spawn? Otherwise the
> > constraint must be something like "before any threads are created in the
> > new process", or something like that?
> 
> The fact that raw fork() may be used in the child created by
> libxl__ev_child_inuse isn't documented.  It would be possible to
> document this but the set of restrictions on the behaviours of the
> middle child and any resulting grandchildren are rather complex.
> 
> I think it might be better to draw a veil over this and leave it as a
> special piece of knowledge implicit in the implementation of
> libxl__spawn_*.

OK.

> In practice the use of _xenstore_reopen in the middle child is fine
> provided that the middle child doesn't then fork _and_ then use
> libxl's xenstore functions in both the middle child and the
> grandchild.
> 
> Ian.



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