[<prev] [next>] [<thread-prev] [day] [month] [year] [list]
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
Powered by blists - more mailing lists