linux-kernel - Re: [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove smp_read_barrier

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Date:   Tue, 10 Oct 2017 10:59:39 +0100
From:   David Howells <dhowells@...hat.com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     dhowells@...hat.com, linux-kernel@...r.kernel.org,
        mingo@...nel.org, torvalds@...ux-foundation.org,
        mark.rutland@....com, linux-arch@...r.kernel.org,
        peterz@...radead.org, will.deacon@....com,
        Jonathan Corbet <corbet@....net>,
        Alexander Kuleshov <kuleshovmail@...il.com>
Subject: Re: [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove smp_read_barrier_depends()

David Howells <dhowells@...hat.com> wrote:

> Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> 
> >  static inline void *assoc_array_ptr_to_leaf(const struct assoc_array_ptr *x)
> >  {
> > -	return (void *)((unsigned long)x & ~ASSOC_ARRAY_PTR_TYPE_MASK);
> > +	return (void *)((unsigned long)READ_ONCE(x) & /* Address dependency. */
> > +		~ASSOC_ARRAY_PTR_TYPE_MASK);
> >  }
> 
> This is the wrong place to do this.  assoc_array_ptr_to_leaf() is effectively
> no more than a special cast; it removes a metadata bit from a pointer.  x is
> the value we're modifying, not *x, and x was read by the caller.

Also, x is not a pointer you can read from, so if READ_ONCE(x) ever effects a
memory access, you might get an oops.

David