Hi Ming,
On Fri, 19 Jun 2020 21:32:40 +0800
Ming Lei <ming.lei@xxxxxxxxxx> wrote:
> On Fri, Jun 19, 2020 at 08:19:54AM -0400, Steven Rostedt wrote:
> > On Fri, 19 Jun 2020 15:28:59 +0800
> > Ming Lei <ming.lei@xxxxxxxxxx> wrote:
> >
> > > >
> > > > OK, then let's make events (for sure)
> > > >
> > > > root@devnote2:/sys/kernel/debug/tracing# echo p __blkdev_put >> kprobe_events
> > > > root@devnote2:/sys/kernel/debug/tracing# echo r __blkdev_put >> kprobe_events
> > > > root@devnote2:/sys/kernel/debug/tracing# echo p blkdev_put >> kprobe_events
> >
> > Hi Ming,
> >
> > Do you have the kprobe_events file?
> >
> > > > root@devnote2:/sys/kernel/debug/tracing# echo 1 > events/kprobes/enable
> > >
> > > I can't find 'events/kprobes' in my VM with upstream kernel, also not found
> > > the dir under fedora31(5.5.15-200) & rhel8(v4.18 based).
> >
> > The events/kprobes directly will be created when you create a
> > kprobe_event. It wont exist until then.
>
> Hi Steven and Masami,
>
> Got it, thanks for your help, now I can run the test, follows the steps
> and results, and there is still one __blkdev_put probed.
Hmm, strange...
> And it is observed in my VM reliably with 5.7+ or Fedora kernel reliably,
> kernel config is attached.
Thanks for sharing it.
>
> [root@ktest-01 tracing]# uname -a
> Linux ktest-01 5.7.0+ #1900 SMP Fri Jun 19 16:26:47 CST 2020 x86_64 x86_64 x86_64 GNU/Linux
> [root@ktest-01 tracing]#
> [root@ktest-01 tracing]# cat kprobe_events
> [root@ktest-01 tracing]#
> [root@ktest-01 tracing]# echo p blkdev_put >> kprobe_events
> [root@ktest-01 tracing]# echo p __blkdev_put >> kprobe_events
> [root@ktest-01 tracing]# echo r __blkdev_put >> kprobe_events
> [root@ktest-01 tracing]#
> [root@ktest-01 tracing]# echo 1 > events/kprobes/enable
> [root@ktest-01 tracing]# blockdev --getbsz /dev/sda1
> 4096
> [root@ktest-01 tracing]# echo 0 > events/kprobes/enable
> [root@ktest-01 tracing]# cat trace
> # tracer: nop
> #
> # entries-in-buffer/entries-written: 3/3 #P:8
> #
> # _-----=> irqs-off
> # / _----=> need-resched
> # | / _---=> hardirq/softirq
> # || / _--=> preempt-depth
> # ||| / delay
> # TASK-PID CPU# |||| TIMESTAMP FUNCTION
> # | | | |||| | |
> blockdev-970 [005] .... 17603.447236: p_blkdev_put_0: (blkdev_put+0x0/0xb4)
> blockdev-970 [005] .... 17603.447244: p___blkdev_put_0: (__blkdev_put+0x0/0x19d)
> blockdev-970 [005] d... 17603.447251: r___blkdev_put_0: (blkdev_close+0x22/0x25 <- __blkdev_put)
This shows __blkdev_put() is a tail-call. It is possible that the
internal (nested) __blkdev_put() call becomes a goto inside the
function by the gcc optimization.
Ah, after all it is as expected. With your kconfig, the kernel is
very agressively optimized.
$ objdump -dS vmlinux | less
...
ffffffff81256dc3 <__blkdev_put>:
{
ffffffff81256dc3: e8 98 85 df ff callq ffffffff8104f360 <__fentry__>
ffffffff81256dc8: 41 57 push %r15
ffffffff81256dca: 41 56 push %r14
ffffffff81256dcc: 41 55 push %r13
...
ffffffff81256f05: 75 02 jne ffffffff81256f09 <__blkdev_put+0x146>
struct block_device *victim = NULL;
ffffffff81256f07: 31 db xor %ebx,%ebx
bdev->bd_contains = NULL;
ffffffff81256f09: 48 c7 45 60 00 00 00 movq $0x0,0x60(%rbp)
ffffffff81256f10: 00
put_disk_and_module(disk);
ffffffff81256f11: 4c 89 f7 mov %r14,%rdi
ffffffff81256f14: e8 c6 3d 11 00 callq ffffffff8136acdf <put_disk_and_module>
mutex_unlock(&bdev->bd_mutex);
ffffffff81256f19: 4c 89 ff mov %r15,%rdi
__blkdev_put(victim, mode, 1);
ffffffff81256f1c: 41 bc 01 00 00 00 mov $0x1,%r12d
mutex_unlock(&bdev->bd_mutex);
ffffffff81256f22: e8 8d d7 48 00 callq ffffffff816e46b4 <mutex_unlock>
bdput(bdev);
ffffffff81256f27: 48 89 ef mov %rbp,%rdi
ffffffff81256f2a: e8 f0 e9 ff ff callq ffffffff8125591f <bdput>
if (victim)
ffffffff81256f2f: 48 85 db test %rbx,%rbx
ffffffff81256f32: 74 08 je ffffffff81256f3c <__blkdev_put+0x179>
ffffffff81256f34: 48 89 dd mov %rbx,%rbp
ffffffff81256f37: e9 b4 fe ff ff jmpq ffffffff81256df0 <__blkdev_put+0x2d> <<-----THIS!!
}
ffffffff81256f3c: 48 8b 44 24 28 mov 0x28(%rsp),%rax
ffffffff81256f41: 65 48 33 04 25 28 00 xor %gs:0x28,%rax
ffffffff81256f48: 00 00
ffffffff81256f4a: 74 05 je ffffffff81256f51 <__blkdev_put+0x18e>
ffffffff81256f4c: e8 5a 4e 48 00 callq ffffffff816dbdab <__stack_chk_fail>
ffffffff81256f51: 48 83 c4 30 add $0x30,%rsp
ffffffff81256f55: 5b pop %rbx
ffffffff81256f56: 5d pop %rbp
ffffffff81256f57: 41 5c pop %r12
ffffffff81256f59: 41 5d pop %r13
ffffffff81256f5b: 41 5e pop %r14
ffffffff81256f5d: 41 5f pop %r15
ffffffff81256f5f: c3 retq
As you can see, the nested __blkdev_put() is coverted to a loop.
If you put kprobe on __blkdev_put+0x2d, you'll see the event twice.
Thank you,
--
Masami Hiramatsu <mhiramat@xxxxxxxxxx>
