Messages in this thread | | | Date | Wed, 11 Oct 2017 09:19:33 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove smp_read_barrier_depends() |
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On Wed, Oct 11, 2017 at 05:07:05PM +0100, David Howells wrote: > Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote: > > > It does not. In most cases, the barriered version would be > > smp_store_release(). > > Ummm... Is that good enough? Is: > > WRITE_ONCE(x, 1); > WRITE_ONCE(x, 2); > > equivalent to: > > smp_store_release(x, 1); > smp_store_release(x, 2); > > if CONFIG_SMP=n?
smp_store_release(&x, 1); smp_store_release(&x, 2);
But yes, give or take that smp_store_release() potentially disables more compiler optimizations than does WRITE_ONCE().
> (Consider what happens if an interrupt messes with x).
OK, I will bite... What is your scenario in which an interrupt gives different results for CONFIG_SMP=n? The barriers
> If it is good enough, should we be using smp_load_acquire() rather than > READ_ONCE()?
On x86, that might be OK, give or take that smp_load_acquire() potentially disables more optimizations than does READ_ONCE(). But on ARM, PowerPC, MIPS, and so on, smp_load_acquire() emits a memory-barrier instruction and READ_ONCE() does not.
Thanx, Paul
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