add abstracted file descriptor based semaphore object that is lock-free in the best cases

git-svn-id: file:///home/lennart/svn/public/pulseaudio/branches/lennart@1537 fefdeb5f-60dc-0310-8127-8f9354f1896f

src/pulsecore/fdsem.c

@@ -0,0 +1,189 @@
+/* $Id$ */
+
+/***
+  This file is part of PulseAudio.
+
+  Copyright 2006 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
+  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 <unistd.h>
+#include <errno.h>
+
+#include <pulsecore/atomic.h>
+#include <pulsecore/log.h>
+#include <pulsecore/thread.h>
+#include <pulsecore/macro.h>
+#include <pulsecore/core-util.h>
+#include <pulse/xmalloc.h>
+
+#include "fdsem.h"
+
+struct pa_fdsem {
+    int fds[2];
+    pa_atomic_t waiting;
+    pa_atomic_t signalled;
+    pa_atomic_t in_pipe;
+};
+
+pa_fdsem *pa_fdsem_new(void) {
+    pa_fdsem *f;
+
+    f = pa_xnew(pa_fdsem, 1);
+    
+    if (pipe(f->fds) < 0) {
+        pa_xfree(f);
+        return NULL;
+    }
+
+    pa_atomic_store(&f->waiting, 0);
+    pa_atomic_store(&f->signalled, 0);
+    pa_atomic_store(&f->in_pipe, 0);
+    
+    return f;
+}
+
+void pa_fdsem_free(pa_fdsem *f) {
+    pa_assert(f);
+
+    pa_assert(pa_atomic_load(&f->waiting) == 0);
+    
+    close(f->fds[0]);
+    close(f->fds[1]);
+
+    pa_xfree(f);
+}
+
+static void flush(pa_fdsem *f) {
+    ssize_t r;
+    pa_assert(f);
+
+    if (pa_atomic_load(&f->in_pipe) <= 0)
+        return;
+
+    do {
+        char x[10];
+        
+        if ((r = read(f->fds[0], &x, sizeof(x))) <= 0) {
+            pa_assert(r < 0 && errno == EINTR);
+            continue;
+        }
+        
+    } while (pa_atomic_sub(&f->in_pipe, r) > r);
+}
+
+void pa_fdsem_post(pa_fdsem *f) {
+    pa_assert(f);
+
+    if (pa_atomic_cmpxchg(&f->signalled, 0, 1)) {
+
+        if (pa_atomic_load(&f->waiting)) {
+            ssize_t r;
+            char x = 'x';
+            
+            pa_atomic_inc(&f->in_pipe);
+
+            for (;;) {
+                
+                if ((r = write(f->fds[1], &x, 1)) != 1) {
+                    pa_assert(r < 0 && errno == EINTR);
+                    continue;
+                }
+
+                break;
+            }
+        }
+    }
+}
+
+void pa_fdsem_wait(pa_fdsem *f) {
+    pa_assert(f);
+
+    flush(f);
+
+    if (pa_atomic_cmpxchg(&f->signalled, 1, 0))
+        return;
+
+    pa_atomic_inc(&f->waiting);
+        
+    while (!pa_atomic_cmpxchg(&f->signalled, 1, 0)) {
+        char x[10];
+        ssize_t r;
+        
+        if ((r = read(f->fds[0], &x, sizeof(x))) <= 0) {
+            pa_assert(r < 0 && errno == EINTR);
+            continue;
+        }
+            
+        pa_atomic_sub(&f->in_pipe, r);
+    }
+
+    pa_atomic_dec(&f->waiting);
+}
+
+int pa_fdsem_try(pa_fdsem *f) {
+    pa_assert(f);
+
+    flush(f);
+    
+    if (pa_atomic_cmpxchg(&f->signalled, 1, 0))
+        return 1;
+
+    return 0;
+}
+
+
+int pa_fdsem_get(pa_fdsem *f) {
+    pa_assert(f);
+    
+    return f->fds[0];
+}
+
+int pa_fdsem_before_poll(pa_fdsem *f) {
+    pa_assert(f);
+
+    flush(f);
+
+    if (pa_atomic_cmpxchg(&f->signalled, 1, 0))
+        return -1;
+
+    pa_atomic_inc(&f->waiting);
+
+    if (pa_atomic_cmpxchg(&f->signalled, 1, 0)) {
+        pa_atomic_dec(&f->waiting);
+        return -1;
+    }
+        
+    return 0;
+}
+
+int pa_fdsem_after_poll(pa_fdsem *f) {
+    pa_assert(f);
+
+    pa_atomic_dec(&f->waiting);
+
+    flush(f);
+
+    if (pa_atomic_cmpxchg(&f->signalled, 1, 0))
+        return 1;
+
+    return 0;
+}

src/pulsecore/fdsem.h

@@ -0,0 +1,49 @@
+#ifndef foopulsefdsemhfoo
+#define foopulsefdsemhfoo
+
+/* $Id$ */
+
+/***
+  This file is part of PulseAudio.
+
+  Copyright 2004-2006 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
+  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.
+***/
+
+#include <sys/types.h>
+#include <pulse/def.h>
+
+/* A simple, asynchronous semaphore which uses fds for sleeping. In
+ * the best case all functions are lock-free unless sleeping is
+ * required.  */
+
+typedef struct pa_fdsem pa_fdsem;
+
+pa_fdsem *pa_fdsem_new(void);
+void pa_fdsem_free(pa_fdsem *f);
+
+void pa_fdsem_post(pa_fdsem *f);
+void pa_fdsem_wait(pa_fdsem *f);
+int pa_fdsem_try(pa_fdsem *f);
+
+int pa_fdsem_get(pa_fdsem *f);
+
+int pa_fdsem_before_poll(pa_fdsem *f);
+int pa_fdsem_after_poll(pa_fdsem *f);
+
+
+#endif