oss: Introduce sound device lock.

Unlike ALSA with dmix, the OSS driver allows only a single opener of
the sound device at any given time. With para_audiod it may happen
that the writer of the buffer tree for the next audio file tries to
open the device while the old writer is still active. If this happens,
the second writer fails to open the device which causes the whole
buffer tree to be aborted.

This patch serializes access to the OSS sound device by adding
a locking mechanism to oss_write.c. Since audiod is single threaded
this is rather simple: The lock is taken in oss_post_select() when
the device is opened. It is released in oss_close() after the file
descriptor has been closed.

diff --git a/oss_write.c b/oss_write.c
index d4bfd446ab845ef1b381201bccf091fadfae96c0..5b910742e3ada1ac084bac89183d3b8e75d03852 100644 (file)
--- a/oss_write.c
+++ b/oss_write.c
@@ -31,6 +31,29 @@ struct private_oss_write_data {
        int bytes_per_frame;
 };
 
+/*
+ * We keep one bit of static storage to make sure only one instance of the oss
+ * writer is running at any given time.
+ */
+static bool sound_device_busy;
+
+static bool sound_device_is_busy(void)
+{
+       return sound_device_busy;
+}
+
+static void set_sound_device_busy(void)
+{
+       assert(!sound_device_busy);
+       sound_device_busy = true;
+}
+
+static void set_sound_device_idle(void)
+{
+       assert(sound_device_busy);
+       sound_device_busy = false;
+}
+
 static int get_oss_format(enum sample_format sf)
 {
        switch (sf) {
@@ -50,7 +73,7 @@ static void oss_pre_select(struct sched *s, void *context)
        struct private_oss_write_data *powd = wn->private_data;
        int ret = btr_node_status(wn->btrn, wn->min_iqs, BTR_NT_LEAF);
 
-       if (ret == 0)
+       if (ret == 0 || (sound_device_is_busy() && !powd))
                return;
        if (ret < 0 || !powd)
                return sched_min_delay(s);
@@ -65,6 +88,7 @@ static void oss_close(struct writer_node *wn)
                return;
        close(powd->fd);
        free(powd);
+       set_sound_device_idle();
 }
 
 /*
@@ -174,12 +198,16 @@ static int oss_post_select(__a_unused struct sched *s, void *context)
                goto out;
        if (!powd) {
                int32_t rate, ch, format;
+
+               if (sound_device_is_busy())
+                       return 0;
                get_btr_sample_rate(btrn, &rate);
                get_btr_channels(btrn, &ch);
                get_btr_sample_format(btrn, &format);
                ret = oss_init(wn, rate, ch, format);
                if (ret < 0)
                        goto out;
+               set_sound_device_busy();
                return 0;
        }
        btr_merge(btrn, wn->min_iqs);