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

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Date:   Mon, 16 Oct 2023 18:55:41 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     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>,
        Ard Biesheuvel <ardb@...nel.org>,
        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 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?

> +retry:
>  	spin_lock_irqsave(&unaccepted_memory_lock, flags);
[...]
> +		spin_unlock_irqrestore(&unaccepted_memory_lock, flags);
> +
> +		/*
> +		 * The code is reachable from atomic context.
> +		 * cond_resched() cannot be used.
> +		 */
> +		cpu_relax();
> +
> +		goto retry;