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Date:   Mon, 18 Sep 2023 10:10:16 +0200
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc:     Jiri Slaby <jirislaby@kernel.org>, Ingo Molnar <mingo@elte.hu>,
        kernel@collabora.com, stable@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org
Subject: Re: [PATCH v3] tty/sysrq: replace smp_processor_id() with get_cpu()
Message-ID: <2023091835-quill-congress-b691@gregkh>
References: <20230822102606.2821311-1-usama.anjum@collabora.com>
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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?

thanks,

greg k-h