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Date: Mon, 2 Dec 2013 18:09:52 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Namhyung Kim <namhyung@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Hyeoncheol Lee <cheol.lee@lge.com>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        "zhangwei(Jovi)" <jovi.zhangwei@huawei.com>,
        Arnaldo Carvalho de Melo <acme@ghostprotocols.net>,
        Hemant Kumar <hkshaw@linux.vnet.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Namhyung Kim <namhyung.kim@lge.com>
Subject: Re: [PATCH 08/17] tracing/probes: Split [ku]probes_fetch_type_table
Message-ID: <20131202170952.GB28115@redhat.com>
References: <1385533203-10014-1-git-send-email-namhyung@kernel.org> <1385533203-10014-9-git-send-email-namhyung@kernel.org> <20131202170454.GA28115@redhat.com>
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On 12/02, Oleg Nesterov wrote:
>
> On 11/27, Namhyung Kim wrote:
> >
> > Use separate fetch_type_table for kprobes and uprobes.  It currently
> > shares all fetch methods but some of them will be implemented
> > differently later.
>
> Hmm. This looks wrong, afaics...
>
> >  static int parse_probe_arg(char *arg, const struct fetch_type *t,
> >  		     struct fetch_param *f, bool is_return, bool is_kprobe)
> >  {
> > +	const struct fetch_type *ftbl;
> >  	unsigned long param;
> >  	long offset;
> >  	char *tmp;
> > -	int ret;
> > +	int ret = 0;
> >
> > -	ret = 0;
> > +	ftbl = is_kprobe ? kprobes_fetch_type_table : uprobes_fetch_type_table;
>
> OK, but what if, say, CONFIG_KPROBE_EVENT && !CONFIG_UPROBE_EVENT ?
> The kernel won't compile in this case?

Ah, wait, probably I was wrong. I didn't noticee that this patch
does

	extern __weak const struct fetch_type kprobes_fetch_type_table[];
	extern __weak const struct fetch_type uprobes_fetch_type_table[];

Is it the reason for "weak" ?

If yes, perhaps this deserves a comment or at least a note in the changelog.
Or is there another reason?

I am wondering if this should always work, with any toolchain. I simply
do not know what is the documented behaviour if a "weak" symbol is never
defined.

Oleg.