[Ada] Fix obscure race condition in term alts

[Arnaud Charlet <charlet at adacore dot com>]

This patch is fixes an obscure race condition in the
implementation of terminate alternatives.
No small test case is available.

Tested on x86_64-pc-linux-gnu, committed on trunk

2011-08-04  Bob Duff  <duff@adacore.com>

	* s-tasren.adb (Task_Do_Or_Queue): Previous code was reading
	Acceptor.Terminate_Alternative without locking Acceptor, which causes a
	race condition whose symptom is to fail to lock Parent. That, in turn,
	causes Parent.Awake_Count to be accessed without locking Parent, which
	causes another race condition whose symptom is that Parent.Awake_Count
	can be off by 1 (either too high or too low). The solution is to lock
	Parent unconditionally, and then lock Acceptor, before reading
	Acceptor.Terminate_Alternative.

Attachment: difs
Description: Text document