linux-kernel - Re: [PATCH RESEND v3] x86/split_lock: fix delayed detection enabling

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Message-ID: <62ba9085-0872-4aab-8871-20d3720a5aee@yandex-team.ru>
Date: Tue, 26 Nov 2024 01:20:39 +0300
From: Maksim Davydov <davydov-max@...dex-team.ru>
To: "Guilherme G. Piccoli" <gpiccoli@...lia.com>
Cc: den-plotnikov@...dex-team.ru, linux-kernel@...r.kernel.org,
 x86@...nel.org, dave.hansen@...ux.intel.com, tglx@...utronix.de,
 mingo@...hat.com, bp@...en8.de
Subject: Re: [PATCH RESEND v3] x86/split_lock: fix delayed detection enabling

Hi!
Thanks a lot!
I've rebased onto the newest master
https://lore.kernel.org/lkml/20241125221147.932377-1-davydov-max@yandex-team.ru/

On 11/20/24 22:16, Guilherme G. Piccoli wrote:
> On 13/11/2024 11:23, Maksim Davydov 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.
>>
>> 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>
>> ---
>>   arch/x86/kernel/cpu/intel.c | 20 ++++++++++++++++----
>>   1 file changed, 16 insertions(+), 4 deletions(-)
> 
> 
> Hi Maksim, thanks for resubmitting again. I think that is indeed a valid
> fix, but what I've also noticed is that recently (as in this week) the
> code changed from the intel.c file to a more generic one, since AMD is
> enabling split lock detection in their CPUs apparently [0].
> 
> So, I'd suggest you to rebase against 6.13-rc, that would likely
> increase the chances of a merge. Once you do that, I can try to test it
> as well, though I don't personally have an Intel CPU with that feature
> (but some friends have it).
> 
> Cheers,
> 
> 
> Guilherme
> 
> 
> [0] https://lore.kernel.org/r/ZzuBNj4JImJGUNJc@gmail.com/

-- 
Best regards,
Maksim Davydov