[COMMITTED] ada: Fix race condition in protected entry call

["Marc Poulhiès" <poulhies@adacore.com>]

From: Ronan Desplanques <desplanques@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