On Mon, Jul 20, 2015 at 02:39:06PM +0100, Will Deacon wrote:
> On Fri, Jul 17, 2015 at 11:14:14PM +0100, Benjamin Herrenschmidt wrote:
> > On Fri, 2015-07-17 at 10:32 +0100, Will Deacon wrote:
> > > static inline void arch_spin_unlock(arch_spinlock_t *lock)
> > > {
> > > - SYNC_IO;
> > > - __asm__ __volatile__("# arch_spin_unlock\n\t"
> > > - PPC_RELEASE_BARRIER: : :"memory");
> > > + smp_mb();
> > > lock->slock = 0;
> > > }
> >
> > That probably needs to be mb() in case somebody has the expectation that
> > it does a barrier vs. DMA on UP.
>
> Hmm, but on !SMP doesn't arch_spin_unlock effectively expand to barrier()
> in the core code?
Yes, to barrier(), but that doesn't generate any code. In contrast, the
mb() that Ben is asking for puts out a sync instruction. Without that
sync instruction, MMIO accesses can be reordered with the spin_unlock(),
even on single-CPU systems. So the bm() is really needed if unlock is
to order against MMIO (and thus DMA) on UP.
Thanx, Paul
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