On Mon, Dec 10, 2018 at 12:32:21AM +0200, Igor Stoppa wrote: > > > On 06/12/2018 11:44, Peter Zijlstra wrote: > > On Wed, Dec 05, 2018 at 03:13:56PM -0800, Andy Lutomirski wrote: > > > > > > + if (op == WR_MEMCPY) > > > > + memcpy((void *)wr_poking_addr, (void *)src, len); > > > > + else if (op == WR_MEMSET) > > > > + memset((u8 *)wr_poking_addr, (u8)src, len); > > > > + else if (op == WR_RCU_ASSIGN_PTR) > > > > + /* generic version of rcu_assign_pointer */ > > > > + smp_store_release((void **)wr_poking_addr, > > > > + RCU_INITIALIZER((void **)src)); > > > > + kasan_enable_current(); > > > > > > Hmm. I suspect this will explode quite badly on sane architectures > > > like s390. (In my book, despite how weird s390 is, it has a vastly > > > nicer model of "user" memory than any other architecture I know > > > of...). I think you should use copy_to_user(), etc, instead. I'm not > > > entirely sure what the best smp_store_release() replacement is. > > > Making this change may also mean you can get rid of the > > > kasan_disable_current(). > > > > If you make the MEMCPY one guarantee single-copy atomicity for native > > words then you're basically done. > > > > smp_store_release() can be implemented with: > > > > smp_mb(); > > WRITE_ONCE(); > > > > So if we make MEMCPY provide the WRITE_ONCE(), all we need is that > > barrier, which we can easily place at the call site and not overly > > complicate our interface with this. > > Ok, so the 3rd case (WR_RCU_ASSIGN_PTR) could be handled outside of this > function. > But, since now memcpy() will be replaced by copy_to_user(), can I assume > that also copy_to_user() will be atomic, if the destination is properly > aligned? On x86_64 it seems yes, however it's not clear to me if this is the > outcome of an optimization or if I can expect it to be always true. This would be a new contraint; one that needs to be documented and verified by the various arch maintainers as they enable this feature on their platform.
