implement a simple lock-free free list

git-svn-id: file:///home/lennart/svn/public/pulseaudio/trunk@1382 fefdeb5f-60dc-0310-8127-8f9354f1896f

src/pulsecore/flist.c

@@ -0,0 +1,226 @@
+/* $Id$ */
+
+/***
+  This file is part of PulseAudio.
+ 
+  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
+  Lesser 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.
+***/
+
+#ifdef HAVE_CONFIG_H
+#include <config.h>
+#endif
+
+#include <assert.h>
+
+#include <pulsecore/atomic.h>
+#include <pulsecore/log.h>
+#include <pulsecore/thread.h>
+#include <pulse/xmalloc.h>
+
+#include "flist.h"
+
+/* Algorithm is not perfect, in a few corner cases it will fail to pop
+ * from the flist although it isn't empty, and fail to push into the
+ * flist, although it isn't full.
+ *
+ * We keep a fixed size array of entries, each item is either marked
+ * UNUSED, USED or BUSY and contains a user data pointer. When pushing
+ * into the queue we look for an UNUSED cell and mark it BUSY with a
+ * CAS operation. If successful we use it and mark it USED, otherwise
+ * we go on and look for the next UNUSED cell. The algorithm for
+ * popping an item from the queue is practically inverse: look for a
+ * USED cell and and mark it BUSY with a CAS operation, after reading
+ * from it mark it UNUSED again.
+ *
+ * To accelerate finding of used/unused cells we maintain a read and a
+ * write index which is used like a ring buffer. After each push we
+ * increase the write index and after each pop we increase the read
+ * index.
+ *
+ * The indexes are incremented atomically and are never truncated to
+ * the buffer size. Instead we assume that the buffer size is a power
+ * of two and that the truncation can thus be done by applying a
+ * simple AND on read.
+ *
+ * To make sure that we do not look for empty cells indefinitely we
+ * maintain a length value which stores the number of used cells. From
+ * this value the number of unused cells is easily calculated. Please
+ * note that the length value is not updated atomically with the read
+ * and write index and might thus be a few cells off the real
+ * value. To deal with this we always look for N_EXTRA_SCAN extra
+ * cells when pushing/popping entries.
+ *
+ * It might make sense to replace this implementation with a link list
+ * stack or queue, which however requires DCAS to be simple. Patches
+ * welcome.
+ *
+ * Please note that this algorithm is home grown.*/
+
+#define FLIST_SIZE 128
+#define N_EXTRA_SCAN 2
+
+/* For debugging purposes we can define _Y to put and extra thread
+ * yield between each operation. */
+
+#ifdef PROFILE
+#define _Y pa_thread_yield()
+#else
+#define _Y do { } while(0)
+#endif
+
+enum {
+    STATE_UNUSED,
+    STATE_USED,
+    STATE_BUSY
+};
+
+struct cell {
+    pa_atomic_int_t state;
+    void *data;
+};
+
+struct pa_flist {
+    struct cell *cells;
+    unsigned size;
+    pa_atomic_int_t length;
+    pa_atomic_int_t read_idx;
+    pa_atomic_int_t write_idx;
+};
+
+static int is_power_of_two(unsigned size) {
+    return !(size & (size - 1));
+}
+
+pa_flist *pa_flist_new(unsigned size) {
+    pa_flist *l;
+
+    if (!size)
+        size = FLIST_SIZE;
+
+    assert(is_power_of_two(size));
+    
+    l = pa_xnew(pa_flist, 1);
+
+    l->size = size;
+    l->cells = pa_xnew0(struct cell, size);
+    
+    pa_atomic_store(&l->read_idx, 0);
+    pa_atomic_store(&l->write_idx, 0);
+    pa_atomic_store(&l->length, 0);
+
+    return l;
+}
+
+static int reduce(pa_flist *l, int value) {
+    return value & (unsigned) (l->size - 1);
+}
+
+void pa_flist_free(pa_flist *l, pa_free_cb_t free_cb) {
+    assert(l);
+
+    if (free_cb) {
+        int len, idx;
+        
+        idx = reduce(l, pa_atomic_load(&l->read_idx));
+        len = pa_atomic_load(&l->length);
+        
+        for (; len > 0; len--) {
+
+            if (pa_atomic_load(&l->cells[idx].state) == STATE_USED)
+                free_cb(l->cells[idx].data);
+
+            idx = reduce(l, idx + 1);
+        }
+    }
+
+    pa_xfree(l->cells);
+    pa_xfree(l);
+}
+
+int pa_flist_push(pa_flist*l, void *p) {
+    int idx, len, n;
+    
+    assert(l);
+    assert(p);
+
+    n = len = (int) l->size - pa_atomic_load(&l->length) + N_EXTRA_SCAN;
+    _Y;
+    idx = reduce(l, pa_atomic_load(&l->write_idx));
+
+    for (; n > 0 ; n--) {
+        _Y;
+
+        if (pa_atomic_cmpxchg(&l->cells[idx].state, STATE_UNUSED, STATE_BUSY)) {
+            _Y;
+            pa_atomic_inc(&l->write_idx);
+            _Y;
+            l->cells[idx].data = p;
+            _Y;
+            pa_atomic_store(&l->cells[idx].state, STATE_USED);
+            _Y;
+            pa_atomic_inc(&l->length);
+            return 0;
+        }
+
+        _Y;
+        idx = reduce(l, idx + 1);
+    }
+
+#ifdef PROFILE
+    if (len > N_EXTRA_SCAN)
+        pa_log("WARNING: Didn't  find free cell after %u iterations.", len);
+#endif
+    
+    return -1;
+}
+
+void* pa_flist_pop(pa_flist*l) {
+    int idx, len, n;
+    
+    assert(l);
+
+    n = len = pa_atomic_load(&l->length) + N_EXTRA_SCAN;
+    _Y;
+    idx = reduce(l, pa_atomic_load(&l->read_idx));
+
+    for (; n > 0 ; n--) {
+        _Y;
+
+        if (pa_atomic_cmpxchg(&l->cells[idx].state, STATE_USED, STATE_BUSY)) {
+            void *p;
+            _Y;
+            pa_atomic_inc(&l->read_idx);
+            _Y;
+            p = l->cells[idx].data;
+            _Y;
+            pa_atomic_store(&l->cells[idx].state, STATE_UNUSED);
+            _Y;
+
+            pa_atomic_dec(&l->length);
+            return p;
+        }
+
+        _Y;
+        idx = reduce(l, idx+1);
+    }
+
+#ifdef PROFILE
+    if (len > N_EXTRA_SCAN)
+        pa_log("WARNING: Didn't find used cell after %u iterations.", len);
+#endif
+    
+    return NULL;
+}

src/pulsecore/flist.h

@@ -0,0 +1,39 @@
+#ifndef foopulseflisthfoo
+#define foopulseflisthfoo
+
+/* $Id$ */
+
+/***
+  This file is part of PulseAudio.
+ 
+  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 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.
+***/
+
+#include <pulse/def.h>
+
+/* A multiple-reader multipler-write lock-free free list implementation */
+
+typedef struct pa_flist pa_flist;
+
+/* Size is required to be a power of two, or 0 for the default size */
+pa_flist * pa_flist_new(unsigned size);
+void pa_flist_free(pa_flist *l, pa_free_cb_t free_cb);
+
+/* Please note that this routine might fail! */
+int pa_flist_push(pa_flist*l, void *p);
+void* pa_flist_pop(pa_flist*l);
+
+#endif