linux-kernel - Re: [PATCH] x86/mm/pat: Fix missing preemption disable for __native_flush

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Date:   Sat, 10 Nov 2018 16:19:08 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        stable <stable@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/mm/pat: Fix missing preemption disable for __native_flush_tlb()


> On Nov 10, 2018, at 3:57 PM, Dan Williams <dan.j.williams@...el.com> wrote:
> 
>> On Fri, Nov 9, 2018 at 4:22 PM Andy Lutomirski <luto@...capital.net> wrote:
>> 
>> 
>> 
>>> On Nov 9, 2018, at 4:05 PM, Dan Williams <dan.j.williams@...el.com> wrote:
>>> 
>>> Commit f77084d96355 "x86/mm/pat: Disable preemption around
>>> __flush_tlb_all()" addressed a case where __flush_tlb_all() is called
>>> without preemption being disabled. It also left a warning to catch other
>>> cases where preemption is not disabled. That warning triggers for the
>>> memory hotplug path which is also used for persistent memory enabling:
>> 
>> I don’t think I agree with the patch. If you call __flush_tlb_all() in a context where you might be *migrated*, then there’s a bug. We could change the code to allow this particular use by checking that we haven’t done SMP init yet, perhaps.
> 
> Hmm, are saying the entire kernel_physical_mapping_init() sequence
> needs to run with pre-emption disabled?

If it indeed can run late in boot or after boot, then it sure looks buggy. Either the __flush_tlb_all() should be removed or it should be replaced with flush_tlb_kernel_range(). It’s unclear to me why a flush is needed at all, but if it’s needed, surely all CPUs need flushing.