linux-kernel - Re: [PATCH v2 04/10] ftrace: Store direct called addresses in their ops

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Message-ID: <CABRcYmJESb9BHpJT9Ty-CCBcsou+CzdZK1myNPPyNvLmw51k3A@mail.gmail.com>
Date:   Thu, 16 Mar 2023 17:15:40 +0100
From:   Florent Revest <revest@...omium.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org, bpf@...r.kernel.org,
        catalin.marinas@....com, will@...nel.org, mhiramat@...nel.org,
        mark.rutland@....com, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org, kpsingh@...nel.org, jolsa@...nel.org,
        xukuohai@...weicloud.com, lihuafei1@...wei.com
Subject: Re: [PATCH v2 04/10] ftrace: Store direct called addresses in their ops

On Thu, Mar 16, 2023 at 4:45 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Thu, 16 Mar 2023 16:40:48 +0100
> Florent Revest <revest@...omium.org> wrote:
>
> > > > @@ -5466,6 +5467,7 @@ __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
> > > >                       entry->direct = addr;
> > > >               }
> > > >       }
> > > > +     WRITE_ONCE(ops->direct_call, addr);
> > >
> > > I'm curious about the use of WRITE_ONCE(). It should not go outside the
> > > mutex barrier.
> >
> > This WRITE_ONCE was originally suggested by Mark here:
> > https://lore.kernel.org/all/Y9vW99htjOphDXqY@FVFF77S0Q05N.cambridge.arm.com/#t
> >
> > My understanding is that it's not so much about avoiding re-ordering
> > but rather about avoiding store tearing since a ftrace_caller
> > trampoline could concurrently read ops->direct_call. Does that make
> > sense ?
>
> Yes, but a comment needs to be added:
>
>      /* Prevent store tearing on some archs */
>      WRITE_ONCE(ops->direct_call, addr);
>
> Or something to that affect. Otherwise I can see it confusing others in the
> future. And probably me too, as I'll forget why it was a WRITE_ONCE() by
> next month. ;-)

Definitely :) I was myself confused after a few weeks of adding it so
I'll add a clarifying comment. Thanks!