linux-kernel - Re: [PATCH] x86/split_lock: Make life miserable for split lockers

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Date:   Tue, 08 Mar 2022 15:59:39 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Fenghua Yu <fenghua.yu@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, patches@...ts.linux.dev
Subject: Re: [PATCH] x86/split_lock: Make life miserable for split lockers

Tony,

On Mon, Mar 07 2022 at 16:37, Tony Luck wrote:
> On Mon, Mar 07, 2022 at 11:30:35PM +0100, Thomas Gleixner wrote:
>> On Wed, Feb 16 2022 at 17:27, Tony Luck wrote:
>> > Questions for this RFC:
>> >
>> > 1) Does this need to be a new option? Maybe just update the
>> >    existing "warn" mode to add this level of extra pain.
>> 
>> That's fine. Warn is the default today, right?
>
> Yes. Warn is the current default.
> Does "That's fine" mean ok to change exiting warn code to add
> this level of pain? Or OK to add a new option?

Add pain to the existing warn code.

>> The question is whether this is something to worry about. If so, then we
>> need to go back to the drawing board.
>
> I don't think it is worth worrying about. The case you describe is
> a process that is about to be preempted when the #AC trap happens.
> In that case this CPU (in fact both HT threads on this core) get
> two jiffies of free split locks.  Cases from here:
>
> 1) The original process gets to run on either of these threads
> before the timeout. They get to execute their split lock and carry
> on running.
>
> 2) The process is scheduled on a different core during the two jiffie
> window. They take an #AC trap and block on the semaphore until the
> original core releases. Then they get their chance to run on this new
> core.
>
> 3) The original process doesn't get rescheduled for two jiffies, then
> runs somewhere. The original core has released the sempahore and re-enabled
> split lock checking. So the process takes #AC, gets the semaphore, kernel
> disables split lock checking ... and we try again.
>
> Now it is possible that the process may repeatedly be preempted in between
> getting the semaphore and actually getting all the way to user space
> to split a lock ... but can only happen if there are multiple processes
> splitting locks. The goal of this patch is to be mean to all of them. If
> we happen to be extra mean to some of them, well so be it.

Fair enough.

I still do not like the inconsistent state between the TIF flag and the
SLD MSR.

Thanks,

        tglx