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

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Message-ID: <1786e05a-300a-4259-be1e-2cc54db9c16b@collabora.com>
Date:   Tue, 22 Aug 2023 15:27:15 +0500
From:   Muhammad Usama Anjum <usama.anjum@...labora.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Muhammad Usama Anjum <usama.anjum@...labora.com>,
        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 8/22/23 2:15 PM, Greg Kroah-Hartman wrote:
> 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...
Sorry, didn't relaize this. I've sent this as v3:
https://lore.kernel.org/all/20230822102606.2821311-1-usama.anjum@collabora.com

> 
> thanks,
> 
> greg k-h

-- 
BR,
Muhammad Usama Anjum