linux-kernel - Re: [RFC][PATCH 5/7] sched,livepatch: Use wake_up_if

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Date:   Thu, 23 Sep 2021 14:19:17 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     Peter Zijlstra <peterz@...radead.org>, gor@...ux.ibm.com,
        jpoimboe@...hat.com, jikos@...nel.org, mingo@...nel.org,
        linux-kernel@...r.kernel.org, joe.lawrence@...hat.com,
        fweisbec@...il.com, tglx@...utronix.de, hca@...ux.ibm.com,
        svens@...ux.ibm.com, sumanthk@...ux.ibm.com,
        live-patching@...r.kernel.org, paulmck@...nel.org
Subject: Re: [RFC][PATCH 5/7] sched,livepatch: Use wake_up_if_idle()

On Wed 2021-09-22 15:05:03, Miroslav Benes wrote:
> > @@ -405,8 +405,10 @@ void klp_try_complete_transition(void)
> >  	for_each_possible_cpu(cpu) {
> >  		task = idle_task(cpu);
> >  		if (cpu_online(cpu)) {
> > -			if (!klp_try_switch_task(task))
> > -				complete = false;
> > +			int ret = klp_try_switch_task(task);
> > +			if (ret == -EBUSY)
> > +				wake_up_if_idle(cpu);
> > +			complete = !ret;
> 
> This is broken. You can basically change "complete" only to false (when it 
> applies). This could leave some tasks in the old patching state.

I was a bit confused by Mirek's comment ;-) Anyway, the following works for me:

@@ -406,9 +406,12 @@ void klp_try_complete_transition(void)
 		task = idle_task(cpu);
 		if (cpu_online(cpu)) {
 			int ret = klp_try_switch_task(task);
-			if (ret == -EBUSY)
-				wake_up_if_idle(cpu);
-			complete = !ret;
+			if (ret) {
+				complete = false;
+				/* Make idle task go through the main loop. */
+				if (ret == -EBUSY)
+					wake_up_if_idle(cpu);
+			}
 		} else if (task->patch_state != klp_target_state) {
 			/* offline idle tasks can be switched immediately */
 			clear_tsk_thread_flag(task, TIF_PATCH_PENDING);

Best Regards,
Petr