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

[Muhammad Usama Anjum]

On 8/22/23 6:24 PM, 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@xxxxxxxxxxxxxxx
>> Fixes: 47cab6a722d4 ("debug lockups: Improve lockup detection, fix generic arch fallback")This commit had introduced the smp_processor_id() function in
sysrq_handle_showallcpus().

> 
> How has this never shown up before now?  What changed to cause this to
> now be triggered?  This feels odd that no one has seen this in the past
> 20+ years :(
Not sure. Probably the combination of reproduction has happened now. The
following three conditions are needed for the warning to appear:
* Enable CONFIG_DEBUG_PREEMPT
* Arch which doesn't define arch_trigger_all_cpu_backtrace such as arm64
* Trigger showallcpu's stack sysrq

> 
> 
>> Signed-off-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
>> ---
>> Changes since v2:
>> - Add changelog and resend
>>
>> Changes since v1:
>> - Add "Cc: stable@xxxxxxxxxxxxxxx" 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());
>>  		if (regs)
>>  			show_regs(regs);
>>  		else
>>  			show_stack(NULL, NULL, KERN_INFO);
>>  
>>  		schedule_work(&sysrq_showallcpus);
>> +		put_cpu();
> 
> Why are you putting the cpu _after_ you schedule the work?
The sysrq_showallcpus work prints stack traces on all CPUs other than the
current CPU. So we are re-enabling preemption after scheduling work from
current CPU. So that it doesn't get changed.
> 
> thanks,
> 
> greg k-h

-- 
BR,
Muhammad Usama Anjum