On Tue, 2018-03-27 at 09:56 +0200, Arnd Bergmann wrote: > On Tue, Mar 27, 2018 at 12:27 AM, Jason Gunthorpe <jgg@xxxxxxxx> wrote: > > On Tue, Mar 27, 2018 at 09:01:57AM +1100, Benjamin Herrenschmidt wrote: > > > On Mon, 2018-03-26 at 17:46 -0400, Sinan Kaya wrote: > > > > I even see patches adding wmb() based on actual observed memory > > corruption during testing on Intel: > > > > https://patchwork.kernel.org/patch/10177207/ > > > > So you think all of this is unnecessary and writel is totally strongly > > ordered, even on multi-socket Intel? > > This example adds a wmb() between two writes to a coherent DMA > area, it is definitely required there. Ah you are right, I incorrectly assumed that the "prod_db" function was an MMIO. So we do NOT have a counter example where wmb is needed on x86, pfiew ! :-) > I'm pretty sure I've never seen > any bug reports pointing to a missing wmb() between memory > and MMIO write accesses, but if you remember seeing them in the > list, maybe you can look again for some evidence of something going > wrong on x86 without it? The interesting thing is that we do seem to have a whole LOT of these spurrious wmb before writel all over the tree, I suspect because of that incorrect recommendation in memory-barriers.txt. We should fix that. Cheers, Ben. -- To unsubscribe from this list: send the line "unsubscribe linux-rdma" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
