linux-kernel - Re: [PATCH] kprobes: Fix to delay the kprobes jump optimization

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Date:   Mon, 22 Feb 2021 11:21:04 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Daniel Axtens <dja@...ens.net>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Michal Hocko <mhocko@...e.com>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH] kprobes: Fix to delay the kprobes jump optimization

On 2021-02-19 10:33:36 [-0800], Paul E. McKenney wrote:
> For definiteness, here is the first part of the change, posted earlier.
> The commit log needs to be updated.  I will post the change that keeps
> the tick going as a reply to this email.
…
> diff --git a/kernel/softirq.c b/kernel/softirq.c
> index 9d71046..ba78e63 100644
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -209,7 +209,7 @@ static inline void invoke_softirq(void)
>  	if (ksoftirqd_running(local_softirq_pending()))
>  		return;
>  
> -	if (!force_irqthreads) {
> +	if (!force_irqthreads || !__this_cpu_read(ksoftirqd)) {
>  #ifdef CONFIG_HAVE_IRQ_EXIT_ON_IRQ_STACK
>  		/*
>  		 * We can safely execute softirq on the current stack if
> @@ -358,8 +358,8 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
>  
>  	pending = local_softirq_pending();
>  	if (pending) {
> -		if (time_before(jiffies, end) && !need_resched() &&
> -		    --max_restart)
> +		if (!__this_cpu_read(ksoftirqd) ||
> +		    (time_before(jiffies, end) && !need_resched() && --max_restart))
>  			goto restart;

This is hunk shouldn't be needed. The reason for it is probably that the
following wakeup_softirqd() would avoid further invoke_softirq()
performing the actual softirq work. It would leave early due to
ksoftirqd_running(). Unless I'm wrong, any raise_softirq() invocation
outside of an interrupt would do the same. 

I would like PeterZ / tglx to comment on this one. Basically I'm not
sure if it is okay to expect softirqs beeing served and waited on that
early in the boot.

>  		wakeup_softirqd();

Sebastian