Re: [PATCH] ipc/sem.c: Update/correct memory barriers.

[Peter Zijlstra]

On Sat, Feb 28, 2015 at 09:36:15PM +0100, Manfred Spraul wrote:
> +/*
> + * Place this after a control barrier (such as e.g. a spin_unlock_wait())
> + * to ensure that reads cannot be moved ahead of the control_barrier.
> + * Writes do not need a barrier, they are not speculated and thus cannot
> + * pass the control barrier.
> + */
> +#ifndef smp_mb__after_control_barrier
> +#define smp_mb__after_control_barrier()	smp_rmb()
> +#endif

Sorry to go bike shedding again; but should we call this:

smp_acquire__after_control_barrier() ?

The thing is; its not a full MB because:

 - stores might actually creep into it; while the control dependency
   guarantees stores will not creep out, nothing is stopping them from
   getting in;

 - its not transitive, and our MB is defined to be so.

Oleg, Paul?
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