Fix purge of code

Ensure that we cannot get any dangling pointers into code that
has been purged. This is done by a two phase purge. At first
phase all fun entries pointing into the code to purge are marked
for purge. All processes trying to call these funs will be suspended
and by this we avoid getting new direct references into the code.
When all processes has been checked, these processes are resumed.

The new purge strategy now also completely ignore the existence of
indirect references to the code (funs). If such exist, they will
cause bad fun exceptions to the caller, but will not prevent a
soft purge or cause a kill of a process having such live references
during a hard purge. This since it is impossible to give any
guarantees that no processes in the system have such indirect
references. Even when the system is completely clean from such
references, new ones can appear via distribution and/or disk.

1 files changed, 23 insertions, 0 deletions

diff --git a/erts/doc/src/erlang.xml b/erts/doc/src/erlang.xml
index 6289f033b2..a146d4f2b1 100644
--- a/erts/doc/src/erlang.xml
+++ b/erts/doc/src/erlang.xml
@@ -764,6 +764,29 @@
             and the operation was requested
             by passing option <c>{allow_gc, false}</c>.</p></item>
         </taglist>
+	<note>
+	<p>
+	  Up until ERTS version 8.*, the check process code operation
+	  checks for all types of references to the old code. That is,
+	  direct references (e.g. return addresses on the process
+	  stack), indirect references (<c>fun</c>s in process
+	  context), and references to literals in the code.
+	</p>
+	<p>
+	  As of ERTS version 9.0, the check process code operation
+	  only checks for direct references to the code. Indirect
+	  references via <c>fun</c>s will be ignored. If such
+	  <c>fun</c>s exist and are used after a purge of the old
+	  code, an exception will be raised upon usage (same as
+	  the case when the <c>fun</c> is received by the process
+	  after the purge). Literals will be taken care of (copied)
+	  at a  later stage. This behavior can as of ERTS version
+	  8.1 be enabled when
+	  <seealso marker="doc/installation_guide:INSTALL#Advanced-configuration-and-build-of-ErlangOTP_Configuring">building OTP</seealso>,
+	  and will automatically be enabled if dirty scheduler
+	  support is enabled.
+	</p>
+	</note>
         <p>See also <seealso marker="kernel:code">code(3)</seealso>.</p>
         <p>Failures:</p>
         <taglist>