linux-kernel - Re: [PATCH v2] mm/gup: fix try_grab_compound_head() race with split_huge

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Message-ID: <2847e923-2366-6a3b-3d76-4513a01981a0@suse.cz>
Date:   Thu, 17 Jun 2021 18:09:16 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Yang Shi <shy828301@...il.com>
Cc:     Jann Horn <jannh@...gle.com>, John Hubbard <jhubbard@...dia.com>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Jan Kara <jack@...e.cz>, stable <stable@...r.kernel.org>
Subject: Re: [PATCH v2] mm/gup: fix try_grab_compound_head() race with
 split_huge_page()

On 6/16/21 8:40 PM, Yang Shi wrote:
> On Wed, Jun 16, 2021 at 10:27 AM Vlastimil Babka <vbabka@...e.cz> wrote:
>>
>> On 6/16/21 1:10 AM, Yang Shi wrote:
>> > On Tue, Jun 15, 2021 at 5:10 AM Jann Horn <jannh@...gle.com> wrote:
>> >>
>> >
>> > It might be possible after Mel's "mm/page_alloc: Allow high-order
>> > pages to be stored on the per-cpu lists" patch
>> > (https://patchwork.kernel.org/project/linux-mm/patch/20210611135753.GC30378@techsingularity.net/).
>>
>> Those would be percpu indeed, but not "lockless, non-atomic", no? They are
>> protected by a local_lock.
> 
> The local_lock is *not* a lock on non-PREEMPT_RT kernel IIUC. It
> disables preempt and IRQ. But preempt disable is no-op on non-preempt
> kernel. IRQ disable can guarantee it is atomic context, but I'm not
> sure if it is equivalent to "atomic freelists" in Jann's context.

Hm right, I thought all locks had the acquire/release semantics, but this one is
just a barrier().