Re: kprobe: __blkdev_put probe is missed

[Masami Hiramatsu]

Hi Ming,

On Thu, 18 Jun 2020 20:54:38 +0800
Ming Lei <ming.lei@xxxxxxxxxx> wrote:

> 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

Could you dump the kprobe_event as below?

# cat /sys/kernel/tracing/kprobe_events


> > 
> > or
> > 
> > /usr/share/bcc/tools/stackcount __blkdev_put
> > 
> > 2) run the following command:
> > blockdev --getbsz /dev/sda1

And dump the kprobe profile?

# cat /sys/kernel/tracing/kprobe_profile

> > 
> > 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.

Could you tell me what kernel are you using?

I'm using 5.4 on ubuntu and can not reproduce it with kprobe_event.

root@devnote2:/sys/kernel/tracing# uname -a
Linux devnote2 5.4.0-37-generic #41-Ubuntu SMP Wed Jun 3 18:57:02 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
root@devnote2:/sys/kernel/tracing# echo p __blkdev_put > kprobe_events 
root@devnote2:/sys/kernel/tracing# echo 1 > events/kprobes/p___blkdev_put_0/enable 
root@devnote2:/sys/kernel/tracing# cat trace
# tracer: nop
#
# entries-in-buffer/entries-written: 0/0   #P:8
#
#                              _-----=> irqs-off
#                             / _----=> need-resched
#                            | / _---=> hardirq/softirq
#                            || / _--=> preempt-depth
#                            ||| /     delay
#           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
#              | |       |   ||||       |         |
root@devnote2:/sys/kernel/tracing# blockdev --getbsz /dev/nvme0n1
4096
root@devnote2:/sys/kernel/tracing# cat trace
# tracer: nop
#
# entries-in-buffer/entries-written: 1/1   #P:8
#
#                              _-----=> irqs-off
#                             / _----=> need-resched
#                            | / _---=> hardirq/softirq
#                            || / _--=> preempt-depth
#                            ||| /     delay
#           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
#              | |       |   ||||       |         |
           <...>-111740 [002] .... 301734.476991: p___blkdev_put_0: (__blkdev_put+0x0/0x1e0)

Hmm, maybe some issue in the latest kernel...?


Thank you,

-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>