From 0efb67b51e0d9d34c65bbdbd9df567a8a991cc4c Mon Sep 17 00:00:00 2001
From: Max Kellermann <max@duempel.org>
Date: Sat, 26 Apr 2014 22:11:23 +0200
Subject: DeferredMonitor: fix race condition when using GLib event loop
Turns out the lock-free code using atomics was not thread-safe. The
given callback could be invoked by GLib before the source_id attribute
was assigned. This commit changes the DeferredMonitor class to use a
Mutex to block the event loop until source_id is assigned. This bug
does not exist in the 0.19 branch because it does not use the GLib
main loop anymore.
---
src/event/DeferredMonitor.cxx | 28 ++++++++++++++++++----------
src/event/DeferredMonitor.hxx | 5 ++++-
2 files changed, 22 insertions(+), 11 deletions(-)
(limited to 'src')
diff --git a/src/event/DeferredMonitor.cxx b/src/event/DeferredMonitor.cxx
index 4ffffaa89..62edb7817 100644
--- a/src/event/DeferredMonitor.cxx
+++ b/src/event/DeferredMonitor.cxx
@@ -27,9 +27,11 @@ DeferredMonitor::Cancel()
#ifdef USE_EPOLL
pending = false;
#else
- const auto id = source_id.exchange(0);
- if (id != 0)
- g_source_remove(id);
+ const ScopeLock protect(mutex);
+ if (source_id != 0) {
+ g_source_remove(source_id);
+ source_id = 0;
+ }
#endif
}
@@ -40,10 +42,9 @@ DeferredMonitor::Schedule()
if (!pending.exchange(true))
fd.Write();
#else
- const unsigned id = loop.AddIdle(Callback, this);
- const auto old_id = source_id.exchange(id);
- if (old_id != 0)
- g_source_remove(old_id);
+ const ScopeLock protect(mutex);
+ if (source_id == 0)
+ source_id = loop.AddIdle(Callback, this);
#endif
}
@@ -65,9 +66,16 @@ DeferredMonitor::OnSocketReady(unsigned)
void
DeferredMonitor::Run()
{
- const auto id = source_id.exchange(0);
- if (id != 0)
- RunDeferred();
+ {
+ const ScopeLock protect(mutex);
+ if (source_id == 0)
+ /* cancelled */
+ return;
+
+ source_id = 0;
+ }
+
+ RunDeferred();
}
gboolean
diff --git a/src/event/DeferredMonitor.hxx b/src/event/DeferredMonitor.hxx
index 2380fb66f..2ac832a0a 100644
--- a/src/event/DeferredMonitor.hxx
+++ b/src/event/DeferredMonitor.hxx
@@ -27,6 +27,7 @@
#include "SocketMonitor.hxx"
#include "WakeFD.hxx"
#else
+#include "thread/Mutex.hxx"
#include <glib.h>
#endif
@@ -48,7 +49,9 @@ class DeferredMonitor
#else
EventLoop &loop;
- std::atomic<guint> source_id;
+ Mutex mutex;
+
+ guint source_id;
#endif
public:
--
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