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Date:   Thu, 4 Jul 2019 14:48:27 +0200
From:   Robert Richter <rric@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     "Frank Ch. Eigler" <fche@redhat.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Jessica Yu <jeyu@kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>, jikos@kernel.org,
        mbenes@suse.cz, Petr Mladek <pmladek@suse.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@redhat.com>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        paulmck <paulmck@linux.ibm.com>,
        "Joel Fernandes, Google" <joel@joelfernandes.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        oprofile-list@lists.sf.net, netdev <netdev@vger.kernel.org>,
        bpf@vger.kernel.org
Subject: Re: [PATCH 2/3] module: Fix up module_notifier return values.
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On 25.06.19 09:42:14, Peter Zijlstra wrote:
> On Mon, Jun 24, 2019 at 04:58:10PM -0400, Frank Ch. Eigler wrote:

> > From peterz's comments, the patches, it's not obvious to me how one is
> > to choose between 0 (NOTIFY_DONE) and 1 (NOTIFY_OK) in the case of a
> > routine success.
> 
> I'm not sure either; what I think I choice was:
> 
>  - if I want to completely ignore the callback, use DONE (per the
>    "Don't care" comment).
> 
>  - if we finished the notifier without error, use OK or
>    notifier_from_errno(0).
> 
> But yes, its a bit of a shit interface.

It looks like it was rarely used in earlier kernels as some sort of
error detection for the notifier calls, e.g.:

1da177e4c3f41524e886b7f1b8a0c1fc7321cac2:kernel/profile.c-int profile_handoff_task(struct task_struct * task)
1da177e4c3f41524e886b7f1b8a0c1fc7321cac2:kernel/profile.c-{
1da177e4c3f41524e886b7f1b8a0c1fc7321cac2:kernel/profile.c-      int ret;
1da177e4c3f41524e886b7f1b8a0c1fc7321cac2:kernel/profile.c-      read_lock(&handoff_lock);
1da177e4c3f41524e886b7f1b8a0c1fc7321cac2:kernel/profile.c-      ret = notifier_call_chain(&task_free_notifier, 0, task);
1da177e4c3f41524e886b7f1b8a0c1fc7321cac2:kernel/profile.c-      read_unlock(&handoff_lock);
1da177e4c3f41524e886b7f1b8a0c1fc7321cac2:kernel/profile.c:      return (ret == NOTIFY_OK) ? 1 : 0;
1da177e4c3f41524e886b7f1b8a0c1fc7321cac2:kernel/profile.c-}

So NOTIFY_OK was used to state there is no error, while NOTIFY_DONE
says the notifier was executed and there might have been errors. The
caller may distinguish the results then.

-Robert