linux-kernel - Re: [PATCH v2 RESEND] tty/sysrq: replace smp_processor_id() with get

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Message-ID: <2023082259-blazer-charity-b58f@gregkh>
Date:   Tue, 22 Aug 2023 11:15:34 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Muhammad Usama Anjum <usama.anjum@...labora.com>
Cc:     Jiri Slaby <jirislaby@...nel.org>, Ingo Molnar <mingo@...e.hu>,
        kernel@...labora.com, stable@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org
Subject: Re: [PATCH v2 RESEND] tty/sysrq: replace smp_processor_id() with
 get_cpu()

On Tue, Aug 22, 2023 at 02:01:11PM +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@...r.kernel.org
> Fixes: 47cab6a722d4 ("debug lockups: Improve lockup detection, fix generic arch fallback")
> Signed-off-by: Muhammad Usama Anjum <usama.anjum@...labora.com>
> ---
> Changes since v1:
> - Add "Cc: stable@...r.kernel.org" tag
> ---

This is not a resend, it is a new version, v3.  Otherwise I would be
confused with the original v2 which is different than this one...

thanks,

greg k-h