From: oliver.upton@xxxxxxxxx > Sent: 16 June 2022 19:45 > > June 16, 2022 11:48 AM, "David Laight" <David.Laight@xxxxxxxxxx> wrote: > > No wonder I was confused. > > It's not surprising the compiler optimises it all away. > > > > It doesn't seem right to be 'abusing' WRITE_ONCE() here. > > Just adding barrier() should be enough and much more descriptive. > > I had the same thought, although I do not believe barrier() is sufficient > on its own. barrier_data() with a pointer to uc passed through > is required to keep clang from eliminating the dead store. A barrier() (full memory clobber) ought to be stronger than the partial one than barrier_data() generates. I can't quite decide whether you need a barrier() both sides of the 'magic write'. Plausibly the compiler could discard the on-stack data after the barrier() and before the 'magic write'. Certainly putting the 'magic write' inside a asm block that has a memory clobber is a more correct solution. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
