From: Qian Cai <cai@lca.pw>
To: Matthew Wilcox <willy@infradead.org>
Cc: akpm@linux-foundation.org, elver@google.com, linux-mm@kvack.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH -next] mm/filemap: fix a data race in filemap_fault()
Date: Mon, 10 Feb 2020 14:20:48 -0500 [thread overview]
Message-ID: <1581362448.7365.38.camel@lca.pw> (raw)
In-Reply-To: <20200210172511.GL8731@bombadil.infradead.org>
On Mon, 2020-02-10 at 09:25 -0800, Matthew Wilcox wrote:
> 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.
Well, I meant to say it is safe from *data* races rather than all other races,
but it is a good catch for the underflow cases and makes some sense to fix them
together (so we don't need to touch the same lines over and over again).
next prev parent reply other threads:[~2020-02-10 19:20 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-10 17:00 [PATCH -next] mm/filemap: fix a data race in filemap_fault() Qian Cai
2020-02-10 17:25 ` Matthew Wilcox
2020-02-10 18:05 ` Kirill A. Shutemov
2020-02-10 19:58 ` Qian Cai
2020-02-10 21:21 ` Kirill A. Shutemov
2020-02-10 19:20 ` Qian Cai [this message]
2020-02-10 19:21 ` Matthew Wilcox
2020-02-10 20:28 ` Qian Cai
2020-02-10 20:44 ` Marco Elver
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