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

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Date:   Fri, 19 Feb 2021 12:04:07 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Ingo Molnar <mingo@...nel.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 Fri, Feb 19, 2021 at 11:45:39AM -0800, Paul E. McKenney wrote:
> On Fri, Feb 19, 2021 at 02:14:29PM -0500, Steven Rostedt wrote:
> > On Fri, 19 Feb 2021 10:18:11 -0800
> > "Paul E. McKenney" <paulmck@...nel.org> wrote:
> > 
> > > We can further prevent entry into dyntick-idle state until
> > > the ksoftirqd kthreads have been spawned, which means that if softirq
> > > handlers must be deferred, they will be resumed within one jiffy by the
> > > next scheduler-clock interrupt.
> > 
> > Why not just prevent entry into dyntick-idle state until the system is
> > finished booting? As you said; There should be no latency-sensitive
> > applications running, until after we started the system.
> 
> Exactly, and that is the effect of patch to rcu_needs_cpu() that I just
> now posted.
> 
> Ah, your point is that if the tick keeps running, there is no need to
> modify softirq?  Good point, and I will test that, thank you!!!

But sadly keeping the tick on without also modifying softirq still
results in a hang.  The problem is that when the kernel is booted with
threadirqs=1, invoke_softirq() will avoid ever running the softirq
handlers on the backside of an interrupt.

So is this where Sebastian tells me that some -rt transformation can
result in locking-based deadlocks if softirq handlers are ever run on
the backside of an interrupt?  ;-)

							Thanx, Paul