linux-kernel - Re: [PATCH 16/17] uprobes: Allocate ->utask before handler

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Message-ID: <20131211181155.GA25144@redhat.com>
Date:	Wed, 11 Dec 2013 19:11:55 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	Namhyung Kim <namhyung@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Hyeoncheol Lee <cheol.lee@....com>,
	"zhangwei(Jovi)" <jovi.zhangwei@...wei.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Hemant Kumar <hkshaw@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Namhyung Kim <namhyung.kim@....com>
Subject: Re: [PATCH 16/17] uprobes: Allocate ->utask before handler_chain()
	for tracing handlers

On 12/11, Masami Hiramatsu wrote:
>
> (2013/12/11 0:57), Oleg Nesterov wrote:
> > On 12/10, Masami Hiramatsu wrote:
> >>
> >> and isn't it better to increment
> >> miss-hit counter of the uprobe?
> >
> > What do you mean? This is not miss-hit and ->utask == NULL is quite normal.
>
> But it could skip the handler_chain silently. It could confuse users
> why their probe doesn't hit as expected.

No, we will restart the same (probed) instruction, handle_swbp()
will be called again, get_utask() will be called again.

Not to mention that (in practice) if GFP_KERNEL fails the task is
already killed.

> > For example, on ppc it can be always NULL because ppc likely emulates the
> > probed insn.
>
> Hmm, in that case, should uprobes handlers never be called on ppc with
> this change?

Why? With this change ppc will have ->utask != NULL even if it doesn't
need it at all.

Oleg.

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