Re: [PATCH] kernel: fix data race in put_pid

[Dmitry Vyukov]

On Fri, Sep 18, 2015 at 1:22 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Fri, Sep 18, 2015 at 10:28:20AM +0100, Will Deacon wrote:
>> On Fri, Sep 18, 2015 at 10:06:46AM +0100, Dmitry Vyukov wrote:
>
>> > Can we have something along the lines of:
>> >
>> > #define atomic_read_ctrl(v) READ_ONCE_CTRL(&(v)->counter)
>>
>> Funnily enough, I had this exact same discussion off-list yesterday
>> afternoon, since I wrote some code relying on a ctrl dependency from
>> an atomic_read to an atomic_xchg_relaxed.
>
> Something a little like so should work fine I suppose.
>
> ---
> Subject: atomic: Implement atomic_read_ctrl()
>
> Provide atomic_read_ctrl() to mirror READ_ONCE_CTRL(), such that we can
> more conveniently use atomics in control dependencies.
>
> Since we can assume atomic_read() implies a READ_ONCE(), we must only
> emit an extra smp_read_barrier_depends() in order to upgrade to
> READ_ONCE_CTRL() semantics.
>
> Requested-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  include/linux/atomic.h | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/include/linux/atomic.h b/include/linux/atomic.h
> index 00a5763e850e..c4072421ea7f 100644
> --- a/include/linux/atomic.h
> +++ b/include/linux/atomic.h
> @@ -4,6 +4,15 @@
>  #include <asm/atomic.h>
>  #include <asm/barrier.h>
>
> +#ifndef atomic_read_ctrl
> +static inline int atomic_read_ctrl(atomic_t *v)
> +{
> +       int val = atomic_read(v);
> +       smp_read_barrier_depends(); /* Enforce control dependency. */
> +       return val;
> +}
> +#endif
> +
>  /*
>   * Relaxed variants of xchg, cmpxchg and some atomic operations.
>   *

Looks good to me.
Should we add atomic64_read_ctrl for completeness? I have not seen
cases where it was needed, though.



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Dmitry Vyukov, Software Engineer, dvyukov@xxxxxxxxxx
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