linux-kernel - Re: [PATCH] kernel/trace: Fix cleanup logic of enable_trace

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Message-ID: <CAPpZLN50yfT7rZxfP6T2g3asE6b8YkAERJHt3UuJ2bh4FAjDCQ@mail.gmail.com>
Date:   Fri, 30 Jun 2023 12:57:36 +0300
From:   Tzvetomir Stoyanov <tz.stoyanov@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     mhiramat@...nel.org, dan.carpenter@...aro.org,
        linux-trace-devel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kernel/trace: Fix cleanup logic of enable_trace_eprobe

On Fri, Jun 30, 2023 at 10:33 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Thu, 29 Jun 2023 17:31:24 +0300
> Tzvetomir Stoyanov <tz.stoyanov@...il.com> wrote:
>
> > On Wed, Jun 28, 2023 at 3:44 PM Steven Rostedt <rostedt@...dmis.org> wrote:
> > >
> > > On Wed, 28 Jun 2023 15:18:11 +0300
> > > "Tzvetomir Stoyanov (VMware)" <tz.stoyanov@...il.com> wrote:
> >> > > --- a/kernel/trace/trace_eprobe.c
> > > > +++ b/kernel/trace/trace_eprobe.c
> > > > @@ -702,8 +702,12 @@ static int enable_trace_eprobe(struct trace_event_call *call,
> > > >
> > > >       if (ret) {
> > > >               /* Failed to enable one of them. Roll back all */
> > > > -             if (enabled)
> > > > -                     disable_eprobe(ep, file->tr);
> > > > +             if (enabled) {
> > >
> > > If one was enabled and the second one failed, that should only happen
> > > if there's a bug in the kernel (unless the failure was due to a memory
> > > problem).
> > >
> > > I wonder if we should add:
> > >
> > >                         int cnt = 0;
> > >
> > > > +                     list_for_each_entry(pos, trace_probe_probe_list(tp), list) {
> > >
> > >                                 /*
> > >                                  * It's a bug if one failed for something other than memory
> > >                                  * not being available but another eprobe succeeded.
> > >                                  */
> > >                                 WARN_ON_ONCE(cnt++ && ret != -ENOMEM);
> >
> > That makes sense, I can send v2 with it. What is the idea of this cnt
> > counter, why not just:
> >                                  WARN_ON_ONCE(ret != -ENOMEM);
> > outside of the loop? If enabled is true and ret is not ENOMEM, the bug
> > is already there.
>
> Failing for something other than ENOMEM should not cause a warning by
> itself. The idea is, if one fails for something other than ENOMEM, they
> all should fail with the same error. That is, they all should succeed
> or they all should fail.
>
> Actually, the above isn't right. The counter should be in the original
> loop, and if one or more succeeds to enable, but another fails due to
> some other error, that needs to be looked at, hence the warning.
>
> Does that make sense?

Yes, it makes sense. But the original loop will break on the first
failure.  If there is an error (ret is not 0) and at least one eprobe
was enabled successfully (enabled is true),
the warning should be emitted, only if that error is not ENOMEM:
                                  WARN_ON_ONCE(ret != -ENOMEM);


>
> -- Steve
>
> > >
> > >
> > > > +                             ep = container_of(pos, struct trace_eprobe, tp);
> > > > +                             disable_eprobe(ep, file->tr);
> > > > +                     }
> > > > +             }
> > > >               if (file)
> > > >                       trace_probe_remove_file(tp, file);
> > > >               else
> > >
> >
> >
>


-- 
Tzvetomir (Ceco) Stoyanov
VMware Open Source Technology Center