linux-kernel - Re: kprobe: __blkdev

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Message-ID: <20200618125438.GA191266@T590>
Date:   Thu, 18 Jun 2020 20:54:38 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Ming Lei <tom.leiming@...il.com>
Cc:     "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-block <linux-block@...r.kernel.org>
Subject: Re: kprobe: __blkdev_put probe is missed

On Wed, Jun 17, 2020 at 06:30:39PM +0800, Ming Lei wrote:
> Hello Guys,
> 
> I found probe on __blkdev_put is missed, which can be observed
> via bcc/perf reliably:
> 
> 1) start trace
> - perf probe __blkdev_put
> - perf trace -a  -e probe:__blkdev_put
> 
> or
> 
> /usr/share/bcc/tools/stackcount __blkdev_put
> 
> 2) run the following command:
> blockdev --getbsz /dev/sda1
> 
> 3) 'perf trace'  or stackcount just  dumps one trace event, and it
> should have been two
> __blkdev_put() traces, since one __blkdev_put() is called for
> partition(/dev/sda1),
> and another is for disk(/dev/sda). If trace_printk() is added in __blkdev_put(),
> two events will be captured from ftrace.
> 

The issue can be shown by loading a kprobe module which registers on
__blkdev_put(), just by replacing _do_fork with __blkdev_put on
samples/kprobes/kprobe_example.c.

So the issue is really in kprobe code.

Thanks,
Ming