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diff --git a/erts/emulator/beam/code_ix.h b/erts/emulator/beam/code_ix.h
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+/*
+ * %CopyrightBegin%
+ *
+ * Copyright Ericsson AB 2012. All Rights Reserved.
+ *
+ * The contents of this file are subject to the Erlang Public License,
+ * Version 1.1, (the "License"); you may not use this file except in
+ * compliance with the License. You should have received a copy of the
+ * Erlang Public License along with this software. If not, it can be
+ * retrieved online at http://www.erlang.org/.
+ *
+ * Software distributed under the License is distributed on an "AS IS"
+ * basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
+ * the License for the specific language governing rights and limitations
+ * under the License.
+ *
+ * %CopyrightEnd%
+ */
+
+/* Description:
+ *	This is the interface that facilitate changing the beam code
+ *      (load,upgrade,delete) while allowing executing Erlang processes to
+ *      access the code without any locks or other expensive memory barriers.
+ *
+ *      The basic idea is to maintain several "logical copies" of the code. These
+ *      copies are identified by a global 'code index', an integer of 0, 1 or 2.
+ *      The code index is used as argument to code access structures like
+ *      export, module, beam_catches, beam_ranges.
+ *
+ *      The current 'active' code index is used to access the current running
+ *      code. The 'staging' code index is used by the process that performs
+ *      a code change operation. When a code change operation completes
+ *      succesfully, the staging code index becomes the new active code index.
+ *
+ *      The third code index is not explicitly used. It can be thought of as
+ *      the "previous active" or the "next staging" index. It is needed to make
+ *      sure that we do not reuse a code index for staging until we are sure
+ *      that no executing BIFs are still referring it.
+ *      We could get by with only two (0 and 1), but that would require that we
+ *      must wait for all schedulers to re-schedule before each code change
+ *      operation can start staging.
+ *
+ *      Note that the 'code index' is very loosely coupled to the concept of
+ *      'current' and 'old' module versions. You can almost say that they are
+ *      orthogonal to each other. Code index is an emulator global concept while
+ *      'current' and 'old' is specific for each module.
+ */
+
+#ifndef __CODE_IX_H__
+#define __CODE_IX_H__
+
+#ifndef __SYS_H__
+#  ifdef HAVE_CONFIG_H
+#    include "config.h"
+#  endif
+#  include "sys.h"
+#endif
+struct process;
+
+
+#define ERTS_NUM_CODE_IX 3
+typedef unsigned ErtsCodeIndex;
+
+
+/* Called once at emulator initialization.
+ */
+void erts_code_ix_init(void);
+
+/* Return active code index.
+ * Is guaranteed to be valid until the calling BIF returns.
+ * To get a consistent view of the code, only one call to erts_active_code_ix()
+ * should be made and the returned ix reused within the same BIF call.
+ */
+ERTS_GLB_INLINE
+ErtsCodeIndex erts_active_code_ix(void);
+
+/* Return staging code ix.
+ * Only used by a process performing code loading/upgrading/deleting/purging.
+ * code_ix must be locked.
+ */
+ERTS_GLB_INLINE
+ErtsCodeIndex erts_staging_code_ix(void);
+
+/* Try seize exclusive code write permission. Needed for code staging.
+ * Main process lock (only) must be held.
+ * System thread progress must not be blocked.
+ * Caller is suspended and *must* yield if 0 is returned. 
+ */
+int erts_try_seize_code_write_permission(struct process*);
+
+/* Release code write permission.
+ * Will resume any suspended waiters.
+ */
+void erts_release_code_write_permission(void);
+
+/* Prepare the "staging area" to be a complete copy of the active code.
+ * Code write permission must have been seized.
+ * Must be followed by calls to either "end" and "commit" or "abort" before
+ * code write permission can be released.
+ */
+void erts_start_staging_code_ix(void);
+
+/* End the staging.
+ * Preceded by "start" and must be followed by "commit".
+ */
+void erts_end_staging_code_ix(void);
+
+/* Set staging code index as new active code index.
+ * Preceded by "end".
+ */
+void erts_commit_staging_code_ix(void);
+
+/* Abort the staging.
+ * Preceded by "start".
+ */
+void erts_abort_staging_code_ix(void);
+
+#ifdef ERTS_ENABLE_LOCK_CHECK
+int erts_has_code_write_permission(void);
+#endif
+
+
+
+#if ERTS_GLB_INLINE_INCL_FUNC_DEF
+
+extern erts_smp_atomic32_t the_active_code_index;
+extern erts_smp_atomic32_t the_staging_code_index;
+
+ERTS_GLB_INLINE ErtsCodeIndex erts_active_code_ix(void)
+{
+    return erts_smp_atomic32_read_nob(&the_active_code_index);
+}
+ERTS_GLB_INLINE ErtsCodeIndex erts_staging_code_ix(void)
+{
+    return erts_smp_atomic32_read_nob(&the_staging_code_index);
+}
+
+#endif /* ERTS_GLB_INLINE_INCL_FUNC_DEF */
+
+#endif /* !__CODE_IX_H__ */
+