On Wed, Feb 19, 2020 at 12:08:40AM -0500, Qian Cai wrote: > > > > On Feb 18, 2020, at 11:52 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote: > > > > If aligned 64bit stores on 64bit host (note the BITS_PER_LONG ifdefs) end up > > being split, the kernel is FUBAR anyway. Details, please - how could that > > end up happening? > > My understanding is the compiler might decide to split the load into saying two 4-byte loads. Then, we might have, > > Load1 > Store > Load2 > > where the load value could be a garbage. Also, Marco (the KCSAN maintainer) who knew more of compiler than me mentioned that there is no guarantee that the store will not be split either. Thus, the WRITE_ONCE(). > I would suggest * if some compiler does that, ask the persons responsible for that "optimization" which flags should be used to disable it. * if they fail to provide such, educate them regarding the usefulness of their idea * if that does not help, don't use the bloody piece of garbage. Again, is that pure theory (because I can't come up with any reason why splitting a 32bit load would be any less legitimate than doing the same to a 64bit one on a 64bit architecture), or is there anything that really would pull that off?
