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Date:   Tue, 14 Sep 2021 09:31:32 +0100
From:   Will Deacon <will@kernel.org>
To:     Marco Elver <elver@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        linux-kernel@vger.kernel.org, kasan-dev@googlegroups.com,
        kernel test robot <oliver.sang@intel.com>
Subject: Re: [PATCH] mm: fix data race in PagePoisoned()
Message-ID: <20210914083132.GA5891@willie-the-truck>
References: <20210913113542.2658064-1-elver@google.com>
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On Mon, Sep 13, 2021 at 01:35:43PM +0200, Marco Elver wrote:
> PagePoisoned() accesses page->flags which can be updated concurrently:
> 
>   | BUG: KCSAN: data-race in next_uptodate_page / unlock_page
>   |
>   | write (marked) to 0xffffea00050f37c0 of 8 bytes by task 1872 on cpu 1:
>   |  instrument_atomic_write           include/linux/instrumented.h:87 [inline]
>   |  clear_bit_unlock_is_negative_byte include/asm-generic/bitops/instrumented-lock.h:74 [inline]
>   |  unlock_page+0x102/0x1b0           mm/filemap.c:1465
>   |  filemap_map_pages+0x6c6/0x890     mm/filemap.c:3057
>   |  ...
>   | read to 0xffffea00050f37c0 of 8 bytes by task 1873 on cpu 0:
>   |  PagePoisoned                   include/linux/page-flags.h:204 [inline]
>   |  PageReadahead                  include/linux/page-flags.h:382 [inline]
>   |  next_uptodate_page+0x456/0x830 mm/filemap.c:2975
>   |  ...
>   | CPU: 0 PID: 1873 Comm: systemd-udevd Not tainted 5.11.0-rc4-00001-gf9ce0be71d1f #1
> 
> To avoid the compiler tearing or otherwise optimizing the access, use
> READ_ONCE() to access flags.
> 
> Link: https://lore.kernel.org/all/20210826144157.GA26950@xsang-OptiPlex-9020/
> Reported-by: kernel test robot <oliver.sang@intel.com>
> Signed-off-by: Marco Elver <elver@google.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
>  include/linux/page-flags.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index a558d67ee86f..628ab237665e 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -206,7 +206,7 @@ static __always_inline int PageCompound(struct page *page)
>  #define	PAGE_POISON_PATTERN	-1l
>  static inline int PagePoisoned(const struct page *page)
>  {
> -	return page->flags == PAGE_POISON_PATTERN;
> +	return READ_ONCE(page->flags) == PAGE_POISON_PATTERN;
>  }

Acked-by: Will Deacon <will@kernel.org>

Thanks for writing up the patch!

Will