On Tue, Dec 17, 2019 at 10:32:35AM -0800, Linus Torvalds wrote: > On Tue, Dec 17, 2019 at 10:04 AM Linus Torvalds > <torva...@linux-foundation.org> wrote: > > > > Let me think about it. > > How about we just get rid of the union entirely, and just use > 'unsigned long' or 'unsigned long long' depending on the size. > > Something like the attached patch - it still requires that it be an > arithmetic type, but now because of the final cast. > > But it might still be a cast to a volatile type, of course. Then the > result will be volatile, but at least now READ_ONCE() won't be taking > the address of a volatile variable on the stack - does that at least > fix some of the horrible code generation. Hmm?
Sounds like it according to mpe, but I'll confirm too for arm64. > This is untested, because I obviously still have the cases of > structures (page table entries) being accessed once.. > > Linus > include/linux/compiler.h | 33 +++++++++++++++++---------------- > 1 file changed, 17 insertions(+), 16 deletions(-) > > diff --git a/include/linux/compiler.h b/include/linux/compiler.h > index 5e88e7e33abe..8b4282194f16 100644 > --- a/include/linux/compiler.h > +++ b/include/linux/compiler.h > @@ -179,18 +179,18 @@ void ftrace_likely_update(struct ftrace_likely_data *f, > int val, > > #include <uapi/linux/types.h> > > -#define __READ_ONCE_SIZE \ > -({ \ > - switch (size) { \ > - case 1: *(__u8 *)res = *(volatile __u8 *)p; break; \ > - case 2: *(__u16 *)res = *(volatile __u16 *)p; break; \ > - case 4: *(__u32 *)res = *(volatile __u32 *)p; break; \ > - case 8: *(__u64 *)res = *(volatile __u64 *)p; break; \ > - default: \ > - barrier(); \ > - __builtin_memcpy((void *)res, (const void *)p, size); \ > - barrier(); \ > - } \ > +/* "unsigned long" or "unsigned long long" - make it fit in a register if > possible */ > +#define __READ_ONCE_TYPE(size) \ > + __typeof__(__builtin_choose_expr(size > sizeof(0UL), 0ULL, 0UL)) Ha, I wondered when '__builtin_choose_expr()' would make an appearance in this thread! Nice trick. I'll try integrating this with what I have and see what I run into next. Back down the rabbit hole... Will