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

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

[Bradley Morgan]

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

Signed-off-by: Bradley Morgan <include@grrlz.net>
---
 kernel/panic.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/kernel/panic.c b/kernel/panic.c
index 03f1eef07b17..8ba3ec12db3a 100644
--- 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
-- 
2.53.0

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

[Andrew Morton]

On Tue,  7 Jul 2026 17:22:52 +0000 Bradley Morgan <include@grrlz.net> 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@grrlz.net

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@linux-foundation.org> wrote:
>On Tue,  7 Jul 2026 17:22:52 +0000 Bradley Morgan <include@grrlz.net>
>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@grrlz.net
>
>

touche, already onto the fix.

I tend to overengineer, this time I UNDERengineered.

V2 is coming 

Thanks!