On Wed, Oct 21, 2015 at 04:04:04PM +0000, David Laight wrote: > From: Paul E. McKenney > > Sent: 21 October 2015 00:35 > ... > > There is also the question of whether the barrier forces ordering > > of unrelated stores, everything initially zero and all accesses > > READ_ONCE() or WRITE_ONCE(): > > > > P0 P1 P2 P3 > > X = 1; Y = 1; r1 = X; r3 = Y; > > some_barrier(); some_barrier(); > > r2 = Y; r4 = X; > > > > P2's and P3's ordering could be globally visible without requiring > > P0's and P1's independent stores to be ordered, for example, if you > > used smp_rmb() for some_barrier(). In contrast, if we used smp_mb() > > for barrier, everyone would agree on the order of P0's and P0's stores. > > > > There are actually a fair number of different combinations of > > aspects of memory ordering. We will need to choose wisely. ;-) > > My thoughts on this are that most code probably isn't performance critical > enough to be using anything other than normal locks for inter-cpu > synchronisation. > Certainly most people are likely to get it wrong somewhere. > So you want a big red sticker saying 'Don't try to be too clever'. I am afraid that I would run out of red stickers rather quickly, given the large number of ways that one can shoot oneself in the foot, even when single-threaded. > Also without examples of why things go wrong (eg member_consumer() > and alpha) it is difficult to understand the differences between > all the barriers (etc). Not just the hardware peculiarities. It is also important to understand the common use cases. > OTOH device driver code may need things slightly stronger than > barrier() (which I think is asm(:::"memory")) to sequence accesses > to hardware devices (and memory the hardware reads), but without > having a strong barrier in every ioread/write() access. There are more memory models than you can shake a stick at, so yes, we do have to choose carefully. And yes, it does get more complex when you add MMIO, and no, I don't know of any formal model that takes MMIO into account. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
