linux-kernel - Re: [PATCHv2] efi/unaccepted: Fix soft lockups caused by parallel memory acceptance

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Message-ID: <CAMj1kXFuCwLbfgymuy6iEf5ztonjkGBn+K42BgSBT_1hQwwrqw@mail.gmail.com>
Date:   Tue, 17 Oct 2023 11:57:06 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joerg Roedel <jroedel@...e.de>,
        Andi Kleen <ak@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Dario Faggioli <dfaggioli@...e.com>,
        Mike Rapoport <rppt@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        marcelo.cerri@...onical.com, tim.gardner@...onical.com,
        philip.cox@...onical.com, aarcange@...hat.com, peterx@...hat.com,
        x86@...nel.org, linux-mm@...ck.org, linux-coco@...ts.linux.dev,
        linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...nel.org, Nikolay Borisov <nik.borisov@...e.com>
Subject: Re: [PATCHv2] efi/unaccepted: Fix soft lockups caused by parallel
 memory acceptance

On Tue, 17 Oct 2023 at 11:44, Kirill A. Shutemov
<kirill.shutemov@...ux.intel.com> wrote:
>
> On Tue, Oct 17, 2023 at 09:42:13AM +0200, Ard Biesheuvel wrote:
> > On Mon, 16 Oct 2023 at 23:39, Kirill A. Shutemov
> > <kirill.shutemov@...ux.intel.com> wrote:
> > >
> > > On Mon, Oct 16, 2023 at 06:55:41PM +0100, Matthew Wilcox wrote:
> > > > On Mon, Oct 16, 2023 at 07:31:22PM +0300, Kirill A. Shutemov wrote:
> > > > >   v2:
> > > > >    - Fix deadlock (Vlastimil);
> > > > >    - Fix comments (Vlastimil);
> > > > >    - s/cond_resched()/cpu_relax()/ -- cond_resched() cannot be called
> > > > >      from atomic context;
> > > >
> > > > Isn't there an implicit cpu_relax() while we're spinning?  Does this
> > > > really accomplish anything?
> > >
> > > You are right. It is useless. I will drop it in v3.
> > >
> >
> > I can drop that bit when applying the patch.
> >
> > One question I have is whether the sequence
> >
> > spin_lock_irqsave(&unaccepted_memory_lock, flags);
> > ...
> > spin_unlock(&unaccepted_memory_lock);
> > arch_accept_memory(phys_start, phys_end);
> > spin_lock(&unaccepted_memory_lock);
> > ...
> > spin_unlock_irqrestore(&unaccepted_memory_lock, flags);
> >
> > is considered sound and is supported by all architectures?
>
> I am not an locking expert and only tested it on x86. But what potential
> issue do you see?
>

Not sure. It just looks slightly out of place, and I am curious
whether all architectures tolerate this asymmetric use.