linux-kernel - Re: [PATCH] mm: fix a data race in put

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Date:   Mon, 10 Feb 2020 07:16:36 -0500
From:   Qian Cai <cai@....pw>
To:     Marco Elver <elver@...gle.com>
Cc:     John Hubbard <jhubbard@...dia.com>, Jan Kara <jack@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>, ira.weiny@...el.com,
        Dan Williams <dan.j.williams@...el.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        kasan-dev <kasan-dev@...glegroups.com>
Subject: Re: [PATCH] mm: fix a data race in put_page()



> On Feb 10, 2020, at 2:48 AM, Marco Elver <elver@...gle.com> wrote:
> 
> Here is an alternative:
> 
> Let's say KCSAN gives you this:
>   /* ... Assert that the bits set in mask are not written
> concurrently; they may still be read concurrently.
>     The access that immediately follows is assumed to access those
> bits and safe w.r.t. data races.
> 
>     For example, this may be used when certain bits of @flags may
> only be modified when holding the appropriate lock,
>     but other bits may still be modified locklessly.
>   ...
>  */
>   #define ASSERT_EXCLUSIVE_BITS(flags, mask)   ....
> 
> Then we can write page_zonenum as follows:
> 
> static inline enum zone_type page_zonenum(const struct page *page)
> {
> +       ASSERT_EXCLUSIVE_BITS(page->flags, ZONES_MASK << ZONES_PGSHIFT);
>        return (page->flags >> ZONES_PGSHIFT) & ZONES_MASK;
> }
> 
> This will accomplish the following:
> 1. The current code is not touched, and we do not have to verify that
> the change is correct without KCSAN.
> 2. We're not introducing a bunch of special macros to read bits in various ways.
> 3. KCSAN will assume that the access is safe, and no data race report
> is generated.
> 4. If somebody modifies ZONES bits concurrently, KCSAN will tell you
> about the race.
> 5. We're documenting the code.
> 
> Anything I missed?

I don’t know. Having to write the same line twice does not feel me any better than data_race() with commenting occasionally.