From: Ronan Desplanques <desplanq...@adacore.com>
This patch only affects the single-entry implementation of protected
objects.
Before this patch, there was a race condition where a task that
called an entry could put itself to sleep right after another task
had executed the entry as a proxy and signalled the not-yet-waiting
first task, which caused the first task to enter a deadlock.
Note that this race condition has been identified and fixed before
for the implementations of the run-time that live under hie/.
This patch reworks the locking sequence so that it is closer to the
one that's used in the multiple-entry implementation of protected
objects. The code for the multiple-entry implementation is spread
across multiple subprograms. To draw a parallel with the section
this patch modifies, one can read the following subprograms:
- System.Tasking.Protected_Objects.Operations.Protected_Entry_Call
- System.Tasking.Entry_Calls.Wait_For_Completion
- System.Tasking.Entry_Calls.Check_Pending_Actions_For_Entry_Call
This patch also adds a comment that explicitly states the locking
constraint that must hold in the affected section.
gcc/ada/
* libgnarl/s-tposen.adb: Fix race condition. Add comment to justify
the locking timing.
Tested on x86_64-pc-linux-gnu, committed on master.
---
gcc/ada/libgnarl/s-tposen.adb | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)
diff --git a/gcc/ada/libgnarl/s-tposen.adb b/gcc/ada/libgnarl/s-tposen.adb
index 9dff6619295..a7447b9e2af 100644
--- a/gcc/ada/libgnarl/s-tposen.adb
+++ b/gcc/ada/libgnarl/s-tposen.adb
@@ -345,11 +345,17 @@ package body
System.Tasking.Protected_Objects.Single_Entry is
pragma Assert (Entry_Call.State /= Cancelled);
+ -- Note that we need to acquire Self_Id's lock before checking the value
+ -- of Entry_Call.State, even though the latter is specified as atomic
+ -- with a pragma. If we didn't, another task could execute the entry on
+ -- our behalf right between the check of Entry_Call.State and the call
+ -- to Wait_For_Completion, and that would cause a deadlock.
+
+ STPO.Write_Lock (Self_Id);
if Entry_Call.State /= Done then
- STPO.Write_Lock (Self_Id);
Wait_For_Completion (Entry_Call'Access);
- STPO.Unlock (Self_Id);
end if;
+ STPO.Unlock (Self_Id);
Check_Exception (Self_Id, Entry_Call'Access);
end Protected_Single_Entry_Call;
--
2.40.0
