posix-cpu-timers: Make expiry_active check actually work correctly

The state tracking changes broke the expiry active check by not writing to
it and instead sitting timers_active, which is already set.

That's not a big issue as the actual expiry is protected by sighand lock,
so concurrent handling is not possible. That means that the second task
which invokes that function executes the expiry code for nothing.

Write to the proper flag.

Also add a check whether the flag is set into check_process_timers(). That
check had been missing in the code before the rework already. The check for
another task handling the expiry of process wide timers was only done in
the fastpath check. If the fastpath check returns true because a per task
timer expired, then the checking of process wide timers was done in
parallel which is as explained above just a waste of cycles.

Fixes: 244d49e306 ("posix-cpu-timers: Move state tracking to struct posix_cputimers")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Frederic Weisbecker <frederic@kernel.org>

kernel/time/posix-cpu-timers.c

@@ -884,16 +884,17 @@ static void check_process_timers(struct task_struct *tsk,
 
 	/*
 	 * If there are no active process wide timers (POSIX 1.b, itimers,
-	 * RLIMIT_CPU) nothing to check.
+	 * RLIMIT_CPU) nothing to check. Also skip the process wide timer
+	 * processing when there is already another task handling them.
 	 */
-	if (!READ_ONCE(pct->timers_active))
+	if (!READ_ONCE(pct->timers_active) || pct->expiry_active)
 		return;
 
-       /*
+	/*
 	 * Signify that a thread is checking for process timers.
 	 * Write access to this field is protected by the sighand lock.
 	 */
-	pct->timers_active = true;
+	pct->expiry_active = true;
 
 	/*
 	 * Collect the current process totals. Group accounting is active