From: Matthew Wilcox <willy@infradead.org>
To: Qian Cai <cai@lca.pw>
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 11:21:55 -0800 [thread overview]
Message-ID: <20200210192155.GM8731@bombadil.infradead.org> (raw)
In-Reply-To: <1581362448.7365.38.camel@lca.pw>
On Mon, Feb 10, 2020 at 02:20:48PM -0500, Qian Cai wrote:
> 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).
My point is that this is a legitimate warning from the sanitiser.
The point of your patches should not be to remove all the warnings!
next prev parent reply other threads:[~2020-02-10 19:22 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
2020-02-10 19:21 ` Matthew Wilcox [this message]
2020-02-10 20:28 ` Qian Cai
2020-02-10 20:44 ` Marco Elver
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