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authorMax Kellermann <max@duempel.org>2013-01-25 23:53:43 +0100
committerMax Kellermann <max@duempel.org>2013-01-25 23:53:43 +0100
commita9b62a2ece7f8b05bbb785edb47be8f37a19284e (patch)
tree690960bb7cc7a72b373b5182ebb9f7beff95d87c /src
parent49567f1f3eab7ac04e4c3c818212e4e3cb437e0f (diff)
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PlayerControl: add second Cond object
This fixes a deadlock bug introduced by 18076ac9. After all, the second Cond was necessary. The problem: two threads can wait for a signal at the same time. The player thread waits for the output thread to finish playback. The main thread waits for the player thread to complete a command. The output thread finishes playback, and sends a signal, which unfortunately does not wake up the player thread, but the main thread. The main thread sees that the command is still not finished, and waits again. The signal is lost forever, and MPD is deadlocked.
Diffstat (limited to 'src')
-rw-r--r--src/PlayerControl.cxx2
-rw-r--r--src/PlayerControl.hxx32
-rw-r--r--src/PlayerThread.cxx2
3 files changed, 34 insertions, 2 deletions
diff --git a/src/PlayerControl.cxx b/src/PlayerControl.cxx
index 3805f776f..01a9c7e81 100644
--- a/src/PlayerControl.cxx
+++ b/src/PlayerControl.cxx
@@ -60,7 +60,7 @@ static void
player_command_wait_locked(struct player_control *pc)
{
while (pc->command != PLAYER_COMMAND_NONE)
- pc->cond.wait(pc->mutex);
+ pc->ClientWait();
}
static void
diff --git a/src/PlayerControl.hxx b/src/PlayerControl.hxx
index 2b25177fb..de05e17ab 100644
--- a/src/PlayerControl.hxx
+++ b/src/PlayerControl.hxx
@@ -108,6 +108,13 @@ struct player_control {
*/
Cond cond;
+ /**
+ * This object gets signalled when the player thread has
+ * finished the #command. It wakes up the client that waits
+ * (i.e. the main thread).
+ */
+ Cond client_cond;
+
enum player_command command;
enum player_state state;
@@ -191,10 +198,35 @@ struct player_control {
* prior to calling this function.
*/
void Wait() {
+ assert(thread == g_thread_self());
+
cond.wait(mutex);
}
/**
+ * Wake up the client waiting for command completion.
+ *
+ * Caller must lock the object.
+ */
+ void ClientSignal() {
+ assert(thread == g_thread_self());
+
+ client_cond.signal();
+ }
+
+ /**
+ * The client calls this method to wait for command
+ * completion.
+ *
+ * Caller must lock the object.
+ */
+ void ClientWait() {
+ assert(thread != g_thread_self());
+
+ client_cond.wait(mutex);
+ }
+
+ /**
* @param song the song to be queued; the given instance will
* be owned and freed by the player
*/
diff --git a/src/PlayerThread.cxx b/src/PlayerThread.cxx
index 23487953f..ac2cb84bd 100644
--- a/src/PlayerThread.cxx
+++ b/src/PlayerThread.cxx
@@ -147,7 +147,7 @@ player_command_finished_locked(struct player_control *pc)
assert(pc->command != PLAYER_COMMAND_NONE);
pc->command = PLAYER_COMMAND_NONE;
- pc->cond.signal();
+ pc->ClientSignal();
}
static void