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

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Message-ID: <20131210155744.GA21466@redhat.com>
Date:	Tue, 10 Dec 2013 16:57:44 +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 12/10, Masami Hiramatsu wrote:
>
> (2013/12/09 15:20), Namhyung Kim wrote:
> > From: Oleg Nesterov <oleg@...hat.com>
> >
> > uprobe_trace_print() and uprobe_perf_print() need to pass the additional
> > info to call_fetch() methods, currently there is no simple way to do this.
> >
> > current->utask looks like a natural place to hold this info, but we need
> > to allocate it before handler_chain().
> >
> > This is a bit unfortunate, perhaps we will find a better solution later,
> > but this is simnple and should work right now.
>
> Hmm, when this will happen?

Perhaps never. Perhaps it will stay forever and we remove get_utask() from
pre_ssout() (it is not needed after this patch).

However I still think we can cleanup this. And to remind, we need to clean
the usage of utask->vaddr in trace_uprobe.c anyway. We can either try to
find another place to pass the info, or we can create a helper(s) for the
tracing handlers to access (and populate if NULL) utask->handler_data.
Note that this (probably) also makes sense because we can unexport
"struct uprobe_task" (but this needs a couple of off-topic cleanups).

We will see. Lets do the minimal change which can work right now, Namhyung
has enough more serious problems ;)

> 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.
For example, on ppc it can be always NULL because ppc likely emulates the
probed insn.

Or did you mean that if get_utask() fails we should report this somehow?
Well, GFP_KERNEL should "never" fail and even if it fails we will restart
the same insn and retry the allocation.

Or did I miss your point completely ?

Oleg.

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