On Mon, Dec 16, 2019 at 10:28:06AM +0000, Will Deacon wrote: > However, enabling this for 32-bit ARM is total carnage; as Linus mentioned, > a whole bunch of code appears to be relying on atomic 64-bit access of > READ_ONCE(); the perf ring buffer, io_uring, the scheduler, pm_runtime, > cpuidle, ... :( > > Unfortunately, at least some of these *do* look like bugs, but I can't see > how we can fix them, not least because the first two are user ABI afaict. It > may also be that in practice we get 2x32-bit stores, and that works out fine > when storing a 32-bit virtual address. I'm not sure what (if anything) the > compiler guarantees in these cases. Perf does indeed have a (known) problem here for the head/tail values. Last time we looked at that nobody could really come up with a sane solution that wouldn't break something. I'll try and dig out that thread. Perhaps casting the value to 'unsigned long' internally might work, I forgot the details.
