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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Tue, 17 Oct 2023 11:57:06 +0200
Message-ID: <CAMj1kXFuCwLbfgymuy6iEf5ztonjkGBn+K42BgSBT_1hQwwrqw@mail.gmail.com>
Subject: Re: [PATCHv2] efi/unaccepted: Fix soft lockups caused by parallel
 memory acceptance
To:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Borislav Petkov <bp@alien8.de>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Sean Christopherson <seanjc@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Joerg Roedel <jroedel@suse.de>,
        Andi Kleen <ak@linux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        David Rientjes <rientjes@google.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Ingo Molnar <mingo@redhat.com>,
        Dario Faggioli <dfaggioli@suse.com>,
        Mike Rapoport <rppt@kernel.org>,
        David Hildenbrand <david@redhat.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        marcelo.cerri@canonical.com, tim.gardner@canonical.com,
        philip.cox@canonical.com, aarcange@redhat.com, peterx@redhat.com,
        x86@kernel.org, linux-mm@kvack.org, linux-coco@lists.linux.dev,
        linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org,
        stable@kernel.org, Nikolay Borisov <nik.borisov@suse.com>
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On Tue, 17 Oct 2023 at 11:44, Kirill A. Shutemov
<kirill.shutemov@linux.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@linux.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.