memtrap,aupdate: split atomic update from memtrap into seperate aupdate framework

src/pulsecore/aupdate.h

@@ -0,0 +1,98 @@
+#ifndef foopulsecoreaupdatehfoo
+#define foopulsecoreaupdatehfoo
+
+/***
+  This file is part of PulseAudio.
+
+  Copyright 2009 Lennart Poettering
+
+  PulseAudio is free software; you can redistribute it and/or modify
+  it under the terms of the GNU Lesser General Public License as
+  published by the Free Software Foundation; either version 2.1 of the
+  License, or (at your option) any later version.
+
+  PulseAudio is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public
+  License along with PulseAudio; if not, write to the Free Software
+  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307
+  USA.
+***/
+
+typedef struct pa_aupdate pa_aupdate;
+
+pa_aupdate *pa_aupdate_new(void);
+void pa_aupdate_free(pa_aupdate *a);
+
+/* Will return 0, or 1, depending on which copy of the data the caller
+ * should look at */
+unsigned pa_aupdate_read_begin(pa_aupdate *a);
+void pa_aupdate_read_end(pa_aupdate *a);
+
+/* Will return 0, or 1, depending which copy of the data the caller
+ * should modify */
+unsigned pa_aupdate_write_begin(pa_aupdate *a);
+void pa_aupdate_write_end(pa_aupdate *a);
+
+/* Will return 0, or 1, depending which copy of the data the caller
+ * should modify. Each time called this will return the opposite of
+ * the previous pa_aupdate_write_begin()/pa_aupdate_write_swap()
+ * call. Should only be called between pa_aupdate_write_begin() and
+ * pa_aupdate_write_end() */
+unsigned pa_aupdate_write_swap(pa_aupdate *a);
+
+/*
+ * This infrastructure allows lock-free updates of arbitrary data
+ * structures in an rcu'ish way: two copies of the data structure
+ * should be exisiting. One side ('the reader') has read access to one
+ * of the two data structure at a time. It does not have to lock it,
+ * however it needs to signal that it is using it/stopped using
+ * it. The other side ('the writer') modifes the second data structure,
+ * and then atomically swaps the two data structures, followed by a
+ * modification of the other one.
+ *
+ * This is intended to be used for cases where the reader side needs
+ * to be fast while the writer side can be slow.
+ *
+ * The reader side is signal handler safe.
+ *
+ * The writer side lock is not recursive. The reader side is.
+ *
+ * There may be multiple readers and multiple writers at the same
+ * time.
+ *
+ * Usage is like this:
+ *
+ * static struct foo bar[2];
+ * static pa_aupdate *a;
+ *
+ * reader() {
+ *     unsigned j;
+ *
+ *     j = pa_update_read_begin(a);
+ *
+ *     ... read the data structure bar[j] ...
+ *
+ *     pa_update_read_end(a);
+ * }
+ *
+ * writer() {
+ *    unsigned j;
+ *
+ *    j = pa_update_write_begin(a);
+ *
+ *    ... update the data structure bar[j] ...
+ *
+ *    j = pa_update_write_swap(a);
+ *
+ *    ... update the data structure bar[j], the same way as above ...
+ *
+ *    pa_update_write_end(a)
+ * }
+ *
+ */
+
+#endif