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

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Message-ID: <CABRcYmLLbRGZXWwEpyLW1YFz87tTPA8pCL7oLd4K6Hp9Etr5LA@mail.gmail.com>
Date:   Thu, 16 Mar 2023 16:40:48 +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 12:43 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Tue,  7 Feb 2023 19:21:29 +0100
> Florent Revest <revest@...omium.org> wrote:
>
> > @@ -5445,6 +5445,7 @@ __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
> >       /* Enable the tmp_ops to have the same functions as the direct ops */
> >       ftrace_ops_init(&tmp_ops);
> >       tmp_ops.func_hash = ops->func_hash;
> > +     tmp_ops.direct_call = addr;
> >
> >       err = register_ftrace_function_nolock(&tmp_ops);
> >       if (err)
> > @@ -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 ?

> -- Steve
>
> >
> >       mutex_unlock(&ftrace_lock);
> >