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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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")

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 :(


> 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());
>  		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?

thanks,

greg k-h