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Date: Fri, 18 Sep 2015 12:30:09 +0100
From: Will Deacon <will.deacon@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Dmitry Vyukov <dvyukov@google.com>, Oleg Nesterov <oleg@redhat.com>,
        "ebiederm@xmission.com" <ebiederm@xmission.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
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        Hans Boehm <hboehm@google.com>
Subject: Re: [PATCH] kernel: fix data race in put_pid
Message-ID: <20150918113009.GD27377@arm.com>
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On Fri, Sep 18, 2015 at 12:22:52PM +0100, Peter Zijlstra 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@google.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  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.
>   *

Works for me:

  Acked-by: Will Deacon <will.deacon@arm.com>

Will