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Date:   Wed, 4 Oct 2023 08:17:31 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Daniel Bristot de Oliveira <bristot@kernel.org>
Cc:     Masami Hiramatsu <mhiramat@kernel.org>,
        linux-trace-kernel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH V3] tracing/timerlat: Hotplug support for the user-space
 interface
Message-ID: <20231004081731.141dfb99@gandalf.local.home>
In-Reply-To: <3fbe67a4-225c-14c1-cb5a-3f667ad80b0e@kernel.org>
References: <a1bbd57692c1a59458c4ee99999b7f83a29bc3c5.1695999408.git.bristot@kernel.org>
        <20231003210309.4335307d@gandalf.local.home>
        <3fbe67a4-225c-14c1-cb5a-3f667ad80b0e@kernel.org>
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On Wed, 4 Oct 2023 12:02:52 +0200
Daniel Bristot de Oliveira <bristot@kernel.org> wrote:

> On 10/4/23 03:03, Steven Rostedt wrote:
> > On Fri, 29 Sep 2023 17:02:46 +0200
> > Daniel Bristot de Oliveira <bristot@kernel.org> wrote:
> >   
> >> The osnoise/per_cpu/CPU$/timerlat_fd is create for each possible
> >> CPU, but it might create confusion if the CPU is not online.
> >>
> >> Create the file only for online CPUs, also follow hotplug by
> >> creating and deleting as CPUs come and go.
> >>
> >> Fixes: e88ed227f639 ("tracing/timerlat: Add user-space interface")  
> > 
> > Is this a fix that needs to go in now and Cc'd to stable? Or is this
> > something that can wait till the next merge window?  
> 
> We can wait for the next merge window... it is a non-trivial fix.
> 

A requirement is if it's a fix, not really how "trivial" it is.

That said, I'm able to consistently triggered this:

BUG: kernel NULL pointer dereference, address: 00000000000000a0
 #PF: supervisor write access in kernel mode
 #PF: error_code(0x0002) - not-present page
 PGD 0 P4D 0 
 Oops: 0002 [#1] PREEMPT SMP PTI
 CPU: 1 PID: 20 Comm: cpuhp/1 Not tainted 6.6.0-rc4-test-00008-g2df8f295b0e2 #103
 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
 RIP: 0010:down_write+0x23/0x70
 Code: 90 90 90 90 90 90 90 f3 0f 1e fa 0f 1f 44 00 00 53 48 89 fb e8 2e bc ff ff bf 01 00 00 00 e8 24 14 31 ff 31 c0 ba 01 00 00 00 <f0> 48 0f b1 13 75 33 65 48 8b 04 25 00 36 03 00 48 89 43 08 bf 01
 RSP: 0018:ffffb17f800e3d98 EFLAGS: 00010246
 RAX: 0000000000000000 RBX: 00000000000000a0 RCX: ffffff8100000000
 RDX: 0000000000000001 RSI: 0000000000000064 RDI: ffffffffb6edd5cc
 RBP: ffffb17f800e3df8 R08: ffff8c6237c61188 R09: 000000008020001b
 R10: ffff8c6237c61160 R11: 0000000000000001 R12: 000000000002da30
 R13: 0000000000000000 R14: ffffffffb6314080 R15: ffff8c6237c61188
 FS:  0000000000000000(0000) GS:ffff8c6237c40000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 00000000000000a0 CR3: 0000000102412001 CR4: 0000000000170ee0
 Call Trace:
  <TASK>
  ? __die+0x23/0x70
  ? page_fault_oops+0x17d/0x4c0
  ? exc_page_fault+0x7f/0x180
  ? asm_exc_page_fault+0x26/0x30
  ? __pfx_osnoise_cpu_die+0x10/0x10
  ? down_write+0x1c/0x70
  ? down_write+0x23/0x70
  ? down_write+0x1c/0x70
  simple_recursive_removal+0xef/0x280
  ? __pfx_remove_one+0x10/0x10
  ? __pfx_osnoise_cpu_die+0x10/0x10
  tracefs_remove+0x44/0x70
  timerlat_rm_per_cpu_interface+0x28/0x70
  osnoise_cpu_die+0xf/0x20
  cpuhp_invoke_callback+0xf8/0x460
  ? __pfx_smpboot_thread_fn+0x10/0x10
  cpuhp_thread_fun+0xf3/0x190
  smpboot_thread_fn+0x18c/0x230
  kthread+0xf7/0x130
  ? __pfx_kthread+0x10/0x10
  ret_from_fork+0x34/0x50
  ? __pfx_kthread+0x10/0x10
  ret_from_fork_asm+0x1b/0x30
  </TASK>
 Modules linked in:
 CR2: 00000000000000a0
 ---[ end trace 0000000000000000 ]---
 RIP: 0010:down_write+0x23/0x70
 Code: 90 90 90 90 90 90 90 f3 0f 1e fa 0f 1f 44 00 00 53 48 89 fb e8 2e bc ff ff bf 01 00 00 00 e8 24 14 31 ff 31 c0 ba 01 00 00 00 <f0> 48 0f b1 13 75 33 65 48 8b 04 25 00 36 03 00 48 89 43 08 bf 01
 RSP: 0018:ffffb17f800e3d98 EFLAGS: 00010246
 RAX: 0000000000000000 RBX: 00000000000000a0 RCX: ffffff8100000000
 RDX: 0000000000000001 RSI: 0000000000000064 RDI: ffffffffb6edd5cc
 RBP: ffffb17f800e3df8 R08: ffff8c6237c61188 R09: 000000008020001b
 R10: ffff8c6237c61160 R11: 0000000000000001 R12: 000000000002da30
 R13: 0000000000000000 R14: ffffffffb6314080 R15: ffff8c6237c61188
 FS:  0000000000000000(0000) GS:ffff8c6237c40000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 00000000000000a0 CR3: 0000000102412001 CR4: 0000000000170ee0
 note: cpuhp/1[20] exited with irqs disabled
 note: cpuhp/1[20] exited with preempt_count 1


With running the attached script as:

 # ./ftrace-test-tracers sleep 1

-- Steve

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