sched/completions: Fix complete_all() semantics

Documentation/scheduler/completion.txt says this about complete_all():

  "calls complete_all() to signal all current and future waiters."

Which doesn't strictly match the current semantics. Currently
complete_all() is equivalent to UINT_MAX/2 complete() invocations,
which is distinctly less than 'all current and future waiters'
(enumerable vs innumerable), although it has worked in practice.

However, Dmitry had a weird case where it might matter, so change
completions to use saturation semantics for complete()/complete_all().
Once done hits UINT_MAX (and complete_all() sets it there) it will
never again be decremented.

Requested-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: der.herr@hofr.at
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/completion.c

@@ -31,7 +31,8 @@ void complete(struct completion *x)
 	unsigned long flags;
 
 	spin_lock_irqsave(&x->wait.lock, flags);
-	x->done++;
+	if (x->done != UINT_MAX)
+		x->done++;
 	__wake_up_locked(&x->wait, TASK_NORMAL, 1);
 	spin_unlock_irqrestore(&x->wait.lock, flags);
 }
@@ -51,7 +52,7 @@ void complete_all(struct completion *x)
 	unsigned long flags;
 
 	spin_lock_irqsave(&x->wait.lock, flags);
-	x->done += UINT_MAX/2;
+	x->done = UINT_MAX;
 	__wake_up_locked(&x->wait, TASK_NORMAL, 0);
 	spin_unlock_irqrestore(&x->wait.lock, flags);
 }
@@ -79,7 +80,8 @@ do_wait_for_common(struct completion *x,
 		if (!x->done)
 			return timeout;
 	}
-	x->done--;
+	if (x->done != UINT_MAX)
+		x->done--;
 	return timeout ?: 1;
 }
 
@@ -280,7 +282,7 @@ bool try_wait_for_completion(struct completion *x)
 	spin_lock_irqsave(&x->wait.lock, flags);
 	if (!x->done)
 		ret = 0;
-	else
+	else if (x->done != UINT_MAX)
 		x->done--;
 	spin_unlock_irqrestore(&x->wait.lock, flags);
 	return ret;