The wmb() after the successfull atomic_cmpxchg() is complete voodoo along
with the comment stating "sync above data before sending IRQ".
There is no "above" data except for the atomic_t stopping_cpu which has
just been acquired. The reboot IPI handler does not check any data and
unconditionally disables the CPU.
Remove this cargo cult barrier.
Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Ashok Raj <[email protected]>
---
arch/x86/kernel/smp.c | 3 ---
1 file changed, 3 deletions(-)
--- a/arch/x86/kernel/smp.c
+++ b/arch/x86/kernel/smp.c
@@ -174,9 +174,6 @@ static void native_stop_other_cpus(int w
atomic_set(&stop_cpus_count, num_online_cpus() - 1);
- /* sync above data before sending IRQ */
- wmb();
-
apic_send_IPI_allbutself(REBOOT_VECTOR);
/*
On Tue, Jun 13, 2023 at 02:17:58PM +0200, Thomas Gleixner wrote:
> The wmb() after the successfull atomic_cmpxchg() is complete voodoo along
> with the comment stating "sync above data before sending IRQ".
>
> There is no "above" data except for the atomic_t stopping_cpu which has
> just been acquired. The reboot IPI handler does not check any data and
> unconditionally disables the CPU.
>
> Remove this cargo cult barrier.
>
> Signed-off-by: Thomas Gleixner <[email protected]>
> Reviewed-by: Ashok Raj <[email protected]>
> ---
> arch/x86/kernel/smp.c | 3 ---
> 1 file changed, 3 deletions(-)
>
> --- a/arch/x86/kernel/smp.c
> +++ b/arch/x86/kernel/smp.c
> @@ -174,9 +174,6 @@ static void native_stop_other_cpus(int w
>
> atomic_set(&stop_cpus_count, num_online_cpus() - 1);
>
> - /* sync above data before sending IRQ */
> - wmb();
> -
> apic_send_IPI_allbutself(REBOOT_VECTOR);
There's a second one a little below. That too should go.
More to the point, the apic_send_*() functions should be the ones that
ensure this if required etc.. See for example weak_wrmsr_fence() for
x2apic.
On Thu, Jun 15 2023 at 10:58, Peter Zijlstra wrote:
> On Tue, Jun 13, 2023 at 02:17:58PM +0200, Thomas Gleixner wrote:
>> The wmb() after the successfull atomic_cmpxchg() is complete voodoo along
>> with the comment stating "sync above data before sending IRQ".
>>
>> There is no "above" data except for the atomic_t stopping_cpu which has
>> just been acquired. The reboot IPI handler does not check any data and
>> unconditionally disables the CPU.
>>
>> Remove this cargo cult barrier.
>>
>> Signed-off-by: Thomas Gleixner <[email protected]>
>> Reviewed-by: Ashok Raj <[email protected]>
>> ---
>> arch/x86/kernel/smp.c | 3 ---
>> 1 file changed, 3 deletions(-)
>>
>> --- a/arch/x86/kernel/smp.c
>> +++ b/arch/x86/kernel/smp.c
>> @@ -174,9 +174,6 @@ static void native_stop_other_cpus(int w
>>
>> atomic_set(&stop_cpus_count, num_online_cpus() - 1);
>>
>> - /* sync above data before sending IRQ */
>> - wmb();
>> -
>> apic_send_IPI_allbutself(REBOOT_VECTOR);
>
> There's a second one a little below. That too should go.
Duh, yes.
> More to the point, the apic_send_*() functions should be the ones that
> ensure this if required etc.. See for example weak_wrmsr_fence() for
> x2apic.
Correct.