linux-kernel - Re: [PATCH v3 0/6] Static calls

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Date:   Fri, 11 Jan 2019 16:55:35 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Nadav Amit <namit@...are.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        David Laight <David.Laight@...lab.com>,
        Borislav Petkov <bp@...en8.de>,
        Julia Cartwright <julia@...com>, Jessica Yu <jeyu@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Edward Cree <ecree@...arflare.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH v3 0/6] Static calls

On Fri, 11 Jan 2019 15:41:22 -0600
Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> > I don’t see RCU-sched solves the problem if you don’t disable preemption. On
> > a fully preemptable kernel, you can get preempted between the push and the
> > call (jmp) or before the push. RCU-sched can then finish, and the preempted
> > task may later jump to a wrong patched-dest.  
> 
> Argh, I misspoke about RCU-sched.  Words are hard.
> 
> I meant synchronize_rcu_tasks(), which is a completely different animal.
> My understanding is that it waits until all runnable tasks (including
> preempted tasks) have gotten a chance to run.

Not quite, but does the same thing. It waits for all tasks to either
schedule voluntarily (not preempted), or be / go into idle, or be /go
into userspace. In any case, it makes sure code is off of trampolines.
I use this before freeing trampolines used by ftrace.

-- Steve