linux-kernel - Re: [PATCH RESEND v4] x86/split_lock: fix delayed detection enabling

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Message-ID: <577f712d-4ac3-481f-8bfa-3d667516f338@yandex-team.ru>
Date: Tue, 18 Mar 2025 23:29:46 +0300
From: Maksim Davydov <davydov-max@...dex-team.ru>
To: Ingo Molnar <mingo@...nel.org>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org,
 den-plotnikov@...dex-team.ru, gpiccoli@...lia.com, tglx@...utronix.de,
 mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com
Subject: Re: [PATCH RESEND v4] x86/split_lock: fix delayed detection enabling



On 2/25/25 16:44, Ingo Molnar wrote:
> 
> * Maksim Davydov <davydov-max@...dex-team.ru> wrote:
> 
>> If the warn mode with disabled mitigation mode is used, then on each
>> CPU where the split lock occurred detection will be disabled in order to
>> make progress and delayed work will be scheduled, which then will enable
>> detection back. Now it turns out that all CPUs use one global delayed
>> work structure. This leads to the fact that if a split lock occurs on
>> several CPUs at the same time (within 2 jiffies), only one CPU will
>> schedule delayed work, but the rest will not. The return value of
>> schedule_delayed_work_on() would have shown this, but it is not checked
>> in the code.
>>
>> A diagram that can help to understand the bug reproduction:
>> https://lore.kernel.org/all/2cd54041-253b-4e78-b8ea-dbe9b884ff9b@yandex-team.ru/
>>
>> In order to fix the warn mode with disabled mitigation mode, delayed work
>> has to be a per-CPU.
>>
>> v4 -> v3:
>> * rebased the patch onto the latest master
>>
>> v3 -> v2:
>> * place and time of the per-CPU structure initialization were changed.
>>    initcall doesn't seem to be a good place for it, so deferred
>>    initialization is used.
>>
>> Fixes: 727209376f49 ("x86/split_lock: Add sysctl to control the misery mode")
>> Signed-off-by: Maksim Davydov <davydov-max@...dex-team.ru>
>> Tested-by: Guilherme G. Piccoli <gpiccoli@...lia.com>
>> ---
>>   arch/x86/kernel/cpu/bus_lock.c | 20 ++++++++++++++++----
>>   1 file changed, 16 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/kernel/cpu/bus_lock.c b/arch/x86/kernel/cpu/bus_lock.c
>> index 704e9241b964..b72235c8db3e 100644
>> --- a/arch/x86/kernel/cpu/bus_lock.c
>> +++ b/arch/x86/kernel/cpu/bus_lock.c
>> @@ -192,7 +192,13 @@ static void __split_lock_reenable(struct work_struct *work)
>>   {
>>   	sld_update_msr(true);
>>   }
>> -static DECLARE_DELAYED_WORK(sl_reenable, __split_lock_reenable);
>> +/*
>> + * In order for each CPU to schedule itself delayed work independently of the
>> + * others, delayed work struct should be per-CPU. This is not required when
>> + * sysctl_sld_mitigate is enabled because of the semaphore, that limits
>> + * the number of simultaneously scheduled delayed works to 1.
>> + */
>> +static DEFINE_PER_CPU(struct delayed_work, sl_reenable);
>>   
>>   /*
>>    * If a CPU goes offline with pending delayed work to re-enable split lock
>> @@ -213,7 +219,7 @@ static int splitlock_cpu_offline(unsigned int cpu)
>>   
>>   static void split_lock_warn(unsigned long ip)
>>   {
>> -	struct delayed_work *work;
>> +	struct delayed_work *work = NULL;
>>   	int cpu;
>>   
>>   	if (!current->reported_split_lock)
>> @@ -235,11 +241,17 @@ static void split_lock_warn(unsigned long ip)
>>   		if (down_interruptible(&buslock_sem) == -EINTR)
>>   			return;
>>   		work = &sl_reenable_unlock;
>> -	} else {
>> -		work = &sl_reenable;
>>   	}
>>   
>>   	cpu = get_cpu();
>> +
>> +	if (!work) {
>> +		work = this_cpu_ptr(&sl_reenable);
>> +		/* Deferred initialization of per-CPU struct */
>> +		if (!work->work.func)
>> +			INIT_DELAYED_WORK(work, __split_lock_reenable);
> 
> The INIT_DELAYED_WORK() call shouldn't be done here, but in some sort
> of init function. The control flow is complicated enough already.
> 
> Also, this_cpu_ptr() isn't necessary, we already have 'cpu' as the
> current CPU. per_cpu() ought to be enough.
> 

Sorry for replying late

I fixed these issues in the new version
https://lore.kernel.org/lkml/20250318144911.10455-1-davydov-max@yandex-team.ru/


> Thanks,
> 
> 	Ingo

-- 
Best regards,
Maksim Davydov