linux-kernel - Re: [PATCH -next] mm/filemap: fix a data race in filemap

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Message-ID: <20200210172511.GL8731@bombadil.infradead.org>
Date:   Mon, 10 Feb 2020 09:25:11 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Qian Cai <cai@....pw>
Cc:     akpm@...ux-foundation.org, elver@...gle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next] mm/filemap: fix a data race in filemap_fault()

On Mon, Feb 10, 2020 at 12:00:29PM -0500, Qian Cai wrote:
> @@ -2622,7 +2622,7 @@ void filemap_map_pages(struct vm_fault *vmf,
>  		if (page->index >= max_idx)
>  			goto unlock;
>  
> -		if (file->f_ra.mmap_miss > 0)
> +		if (data_race(file->f_ra.mmap_miss > 0))
>  			file->f_ra.mmap_miss--;

How is this safe?  Two threads can each see 1, and then both decrement the
in-memory copy, causing it to end up at -1.