On Thu, Jul 06, 2023 at 12:23:57AM +0200, Frederic Weisbecker wrote:

Le Wed, Jul 05, 2023 at 07:12:53PM +0100, Valentin Schneider a écrit :
+bool ct_set_cpu_work(unsigned int cpu, unsigned int work)
+{
+	struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu);
+	unsigned int old, new, state;
+	bool ret = false;
+
+	preempt_disable();
+
+	work <<= CONTEXT_WORK;
+	state = atomic_read(&ct->state);
+	/*
+	 * Try setting the work until either
+	 * - the target CPU is on the kernel
+	 * - the work has been set
+	 */
+	for (;;) {
+		/* Only set if running in user/guest */
+		old = state;
+		old &= ~CONTEXT_MASK;
+		old |= CONTEXT_USER;
+
+		new = old | work;
+
+		state = atomic_cmpxchg(&ct->state, old, new);
+		if (state & work) {

And this should be "if (state == old)", otherwise there is
a risk that someone else had set the work but atomic_cmpxchg()
failed due to other modifications is the meantime. It's then
dangerous in that case to defer the work because atomic_cmpxchg()
failures don't imply full ordering. So there is a risk that the
target executes the work but doesn't see the most recent data.

Thanks.