Re: [PATCH] panic: stop CPUs that lose the panic_redirect_cpu race

[Bradley Morgan]

On July 7, 2026 7:12:56 PM GMT+01:00, Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>On Tue,  7 Jul 2026 17:22:52 +0000 Bradley Morgan <include@xxxxxxxxx>
>wrote:
>
>> Loser of the redirect cmpxchg must stop, not fall through to
>> panic_try_start().
>> 
>> This issue was found by sashiko [1].
>> 
>> [1]
>https://sashiko.dev/#/patchset/20260705164123.18746-1-include%40grrlz.net
>> 
>> ...
>>
>> --- a/kernel/panic.c
>> +++ b/kernel/panic.c
>> @@ -400,12 +400,9 @@ static bool panic_try_force_cpu(const char *fmt,
>va_list args)
>>  	if (panic_in_progress())
>>  		return false;
>>  
>> -	/*
>> -	 * Only one CPU can do the redirect. Use atomic cmpxchg to ensure
>> -	 * we don't race with another CPU also trying to redirect.
>> -	 */
>> +	/* Only one CPU redirects, the loser stops and lets it finish. */
>>  	if (!atomic_try_cmpxchg(&panic_redirect_cpu, &old_cpu, this_cpu))
>> -		return false;
>> +		return true;
>>  
>>  	/*
>>  	 * Use dynamically allocated buffer if available, otherwise
>
>Sashiko is at it again:
>	https://sashiko.dev/#/patchset/20260707172252.4842-1-include@xxxxxxxxx
>
>

touche, already onto the fix.

I tend to overengineer, this time I UNDERengineered.

V2 is coming 

Thanks!