On Thu, Feb 14, 2019 at 10:44:49AM +0000, Alexey Brodkin wrote: > > On Wed, Feb 13, 2019 at 03:23:36PM -0800, Vineet Gupta wrote: > > > On 2/13/19 4:56 AM, Peter Zijlstra wrote: > > > > > > > > Personally I think u64 and company should already force natural > > > > alignment; but alas. > > > > > > But there is an ISA/ABI angle here too. e.g. On 32-bit ARC, LDD (load double) is > > > allowed to take a 32-bit aligned address to load a register pair. Thus all u64 > > > need not be 64-bit aligned (unless attribute aligned 8 etc) hence the relaxation > > > in ABI (alignment of long long is 4). You could certainly argue that we end up > > > undoing some of it anyways by defining things like ARCH_KMALLOC_MINALIGN to 8, but > > > still... > > > > So what happens if the data is then split across two cachelines; will a > > STD vs LDD still be single-copy-atomic? I don't _think_ we rely on that > > for > sizeof(unsigned long), with the obvious exception of atomic64_t, > > but yuck... > > STD & LDD are simple store/load instructions so there's no problem for > their 64-bit data to be from 2 subsequent cache lines as well as 2 pages > (if we're that unlucky). Or you mean something else? u64 x; WRITE_ONCE(x, 0x1111111100000000); WRITE_ONCE(x, 0x0000000011111111); vs t = READ_ONCE(x); is t allowed to be 0x1111111111111111 ? If the data is split between two cachelines, the hardware must do something very funny to avoid that. single-copy-atomicity requires that to never happen; IOW no load or store tearing. You must observe 'whole' values, no mixing. Linux requires READ_ONCE()/WRITE_ONCE() to be single-copy-atomic for <=sizeof(unsigned long) and atomic*_read()/atomic*_set() for all atomic types. Your atomic64_t alignment should ensure this is so. So while I think we're fine, I do find hardware instructions that tear yuck (yah, I know, x86...) > > So even though it is allowed by the chip; does it really make sense to > > use this? > > It gives performance benefits when dealing with either 64-bit or even > larger buffers, see how we use it in our string routines like here [1]. > > [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arc/lib/memset-archs.S#n81 That doesn't require the ABI alignment crud.
