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SubjectRe: 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

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