Re: kprobe: __blkdev_put probe is missed

[Ming Lei <ming.lei@redhat.com>]

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