Quoting Samuel Thibault (2014-05-23 01:40:19)
> Justus Winter, le Tue 13 May 2014 21:02:56 +0200, a écrit :
> > diff --git a/libtrivfs/protid-clean.c b/libtrivfs/protid-clean.c
> > index f98da6a..cce736d 100644
> > --- a/libtrivfs/protid-clean.c
> > +++ b/libtrivfs/protid-clean.c
> > @@ -31,19 +31,23 @@ trivfs_clean_protid (void *arg)
> > (*trivfs_protid_destroy_hook) (cred);
> >
> > /* If we hold the only reference to the peropen, try to get rid of it. */
> > - pthread_mutex_lock (&cntl->lock);
> > - if (cred->po->refcnt == 1 && trivfs_peropen_destroy_hook)
> > + if (refcount_deref (&cred->po->refcnt) == 0)
> > {
> > - pthread_mutex_unlock (&cntl->lock);
> > - (*trivfs_peropen_destroy_hook) (cred->po);
> > - pthread_mutex_lock (&cntl->lock);
> > + if (trivfs_peropen_destroy_hook)
> > + {
> > + /* Reaquire a reference while we call the hook. */
> > + refcount_ref (&cred->po->refcnt);
> > + (*trivfs_peropen_destroy_hook) (cred->po);
> > + if (refcount_deref (&cred->po->refcnt) == 0)
> > + goto free_po;
> > + }
> > + else
> > + {
> > + free_po:
> > + ports_port_deref (cntl);
> > + free (cred->po);
> > + }
>
> Why not doing it as before: check whether it's the last reference,
> instead of unreferencing, then re-referencing for the hook, and
> unreference again?
It is an optimization. This way we can use just 1 atomic operation
when trivfs_peropen_destroy_hook is not used, and 3 when it is. If we
do it like you described, we need two in any case.
A quick survey of the Hurd codebase revealed that trivfs-based
translators *not* using trivfs_peropen_destroy_hook are exec, pfinet
and pflocal. Translators using it are storeio, term, fifo, hello*,
magic, mtab, new-fifo, streamio.
Justus
