Re: [PATCH v3] tty/sysrq: replace smp_processor_id() with get_cpu()

[Muhammad Usama Anjum <usama.anjum@collabora.com>]

On 9/19/23 12:13 PM, Greg Kroah-Hartman wrote:
> On Tue, Sep 19, 2023 at 07:52:42AM +0200, Jiri Slaby wrote:
>> On 18. 09. 23, 10:10, Greg Kroah-Hartman wrote:
>>> On Tue, Aug 22, 2023 at 03:26:06PM +0500, Muhammad Usama Anjum wrote:
>>>> The smp_processor_id() shouldn't be called from preemptible code.
>>>> Instead use get_cpu() and put_cpu() which disables preemption in
>>>> addition to getting the processor id. This fixes the following bug:
>>>>
>>>> [  119.143590] sysrq: Show backtrace of all active CPUs
>>>> [  119.143902] BUG: using smp_processor_id() in preemptible [00000000] code: bash/873
>>>> [  119.144586] caller is debug_smp_processor_id+0x20/0x30
>>>> [  119.144827] CPU: 6 PID: 873 Comm: bash Not tainted 5.10.124-dirty #3
>>>> [  119.144861] Hardware name: QEMU QEMU Virtual Machine, BIOS 2023.05-1 07/22/2023
>>>> [  119.145053] Call trace:
>>>> [  119.145093]  dump_backtrace+0x0/0x1a0
>>>> [  119.145122]  show_stack+0x18/0x70
>>>> [  119.145141]  dump_stack+0xc4/0x11c
>>>> [  119.145159]  check_preemption_disabled+0x100/0x110
>>>> [  119.145175]  debug_smp_processor_id+0x20/0x30
>>>> [  119.145195]  sysrq_handle_showallcpus+0x20/0xc0
>>>> [  119.145211]  __handle_sysrq+0x8c/0x1a0
>>>> [  119.145227]  write_sysrq_trigger+0x94/0x12c
>>>> [  119.145247]  proc_reg_write+0xa8/0xe4
>>>> [  119.145266]  vfs_write+0xec/0x280
>>>> [  119.145282]  ksys_write+0x6c/0x100
>>>> [  119.145298]  __arm64_sys_write+0x20/0x30
>>>> [  119.145315]  el0_svc_common.constprop.0+0x78/0x1e4
>>>> [  119.145332]  do_el0_svc+0x24/0x8c
>>>> [  119.145348]  el0_svc+0x10/0x20
>>>> [  119.145364]  el0_sync_handler+0x134/0x140
>>>> [  119.145381]  el0_sync+0x180/0x1c0
>>>>
>>>> Cc: stable@vger.kernel.org
>>>> Fixes: 47cab6a722d4 ("debug lockups: Improve lockup detection, fix generic arch fallback")
>>>> Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
>>>> ---
>>>> Changes since v2:
>>>> - Add changelog and resend
>>>>
>>>> Changes since v1:
>>>> - Add "Cc: stable@vger.kernel.org" tag
>>>> ---
>>>>   drivers/tty/sysrq.c | 3 ++-
>>>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
>>>> index 23198e3f1461a..6b4a28bcf2f5f 100644
>>>> --- a/drivers/tty/sysrq.c
>>>> +++ b/drivers/tty/sysrq.c
>>>> @@ -262,13 +262,14 @@ static void sysrq_handle_showallcpus(u8 key)
>>>>   		if (in_hardirq())
>>>>   			regs = get_irq_regs();
>>>> -		pr_info("CPU%d:\n", smp_processor_id());
>>>> +		pr_info("CPU%d:\n", get_cpu());
>>>
>>> Why not call put_cpu() right here?
>>>
>>>>   		if (regs)
>>>>   			show_regs(regs);
>>>>   		else
>>>>   			show_stack(NULL, NULL, KERN_INFO);
>>>>   		schedule_work(&sysrq_showallcpus);
>>>> +		put_cpu();
>>>
>>> Why wait so long here after you have scheduled work?  Please drop the
>>> cpu reference right away, you don't need to hold it for this length of
>>> time, right?
>>
>> As I understand it, this way, schedule_work() will queue the work on the
>> "gotten" (current) CPU. So sysrq_showregs_othercpus() will really dump other
>> than the "gotten" cpu.
> 
> Ok, that makes a bit more sense, but that's not what the code does
> today, have people seen the regs dumped from the wrong cpu in the past?
> 
>> If that is the case, it indeed should have been described in the commit log.
Thanks for review. I'll add the explanation in the commit log and send again.

> 
> Agreed.
> 
> thanks for the review,
> 
> greg k-h

-- 
BR,
Muhammad Usama Anjum