This patch fixes a subtle race condition where the master task and the child
task may free the child task concurrently. Found by code reading.
Tested on x86_64-pc-linux-gnu, committed on trunk
2012-10-05 Arnaud Charlet <[email protected]>
* s-tassta.adb: Update comments.
(Vulnerable_Complete_Master): If Free_On_Termination is set, do
nothing, and let the task free itself if not already done.
Index: s-tassta.adb
===================================================================
--- s-tassta.adb (revision 192066)
+++ s-tassta.adb (working copy)
@@ -1905,7 +1905,16 @@
C := All_Tasks_List;
P := null;
while C /= null loop
- if C.Common.Parent = Self_ID and then C.Master_of_Task >= CM then
+ -- If Free_On_Termination is set, do nothing here, and let
+ -- the task free itself if not already done, otherwise we
+ -- risk a race condition where Vulnerable_Free_Task is called
+ -- in the loop below, while the task calls Free_Task itself,
+ -- in Terminate_Task.
+
+ if C.Common.Parent = Self_ID
+ and then C.Master_of_Task >= CM
+ and then not C.Free_On_Termination
+ then
if P /= null then
P.Common.All_Tasks_Link := C.Common.All_Tasks_Link;
else
@@ -2088,9 +2097,7 @@
-- is called from Expunge_Unactivated_Tasks.
-- For tasks created by elaboration of task object declarations it is
- -- called from the finalization code of the Task_Wrapper procedure. It is
- -- also called from Ada.Unchecked_Deallocation, for objects that are or
- -- contain tasks.
+ -- called from the finalization code of the Task_Wrapper procedure.
procedure Vulnerable_Free_Task (T : Task_Id) is
begin
