diff -r c3f280acf41a tools/ioemu/block-raw.c
--- a/tools/ioemu/block-raw.c	Mon Jun 18 16:59:06 2007 +0100
+++ b/tools/ioemu/block-raw.c	Wed Jun 20 12:53:21 2007 +0800
@@ -166,7 +166,7 @@ typedef struct RawAIOCB {
     struct RawAIOCB *next;
 } RawAIOCB;
 
-static int aio_sig_num = SIGUSR2;
+int aio_sig_num = SIGUSR2;
 static RawAIOCB *first_aio; /* AIO issued */
 static int aio_initialized = 0;
 
diff -r c3f280acf41a tools/ioemu/vl.c
--- a/tools/ioemu/vl.c	Mon Jun 18 16:59:06 2007 +0100
+++ b/tools/ioemu/vl.c	Wed Jun 20 13:55:26 2007 +0800
@@ -7059,6 +7059,19 @@ int main(int argc, char **argv)
 #endif
 
     char qemu_dm_logfilename[128];
+    
+    /* The Qemu we're using is actually multi-threaded! this code block
+       ensures the SIGUSR2 is blocked by default when a new thread is
+       created, then only the threads that use the signal unblock it -- this
+       fixes a race condition in Qcow.  */
+    {
+        extern int aio_sig_num;
+        sigset_t set;
+
+        sigemptyset(&set);
+        sigaddset(&set, aio_sig_num);
+        sigprocmask(SIG_BLOCK, &set, NULL);
+    }
 
     LIST_INIT (&vm_change_state_head);
 #ifndef _WIN32