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