[pypy-commit] stmgc default: Add a comment that finally settles my mind

[arigo]

Author: Armin Rigo <arigo at tunes.org>
Branch: 
Changeset: r429:d633d35aa61e
Date: 2013-07-21 09:29 +0200
http://bitbucket.org/pypy/stmgc/changeset/d633d35aa61e/

Log:	Add a comment that finally settles my mind

diff --git a/c4/nursery.c b/c4/nursery.c
--- a/c4/nursery.c
+++ b/c4/nursery.c
@@ -235,6 +235,12 @@
         /* the backup copy is always allocated outside the nursery,
            but we have to trace it as well, as it may contain its own
            young pointers.
+           
+           but only once: if the transaction was running for long enough
+           to have num_private_from_protected_known_old > 0, then the
+           backup copies of known-old objects have already been traced
+           in a previous minor collection, and as they are read-only,
+           they cannot contain young pointers any more.
         */
         stmgc_trace((gcptr)items[i]->h_revision, &visit_if_young);
     }