Messages in this thread | | | Date | Thu, 2 Jul 2020 12:18:33 +0100 | From | Will Deacon <> | Subject | Re: [PATCH 04/18] alpha: Override READ_ONCE() with barriered implementation |
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On Thu, Jul 02, 2020 at 12:08:41PM +0200, Arnd Bergmann wrote: > On Thu, Jul 2, 2020 at 11:48 AM Will Deacon <will@kernel.org> wrote: > > On Thu, Jul 02, 2020 at 10:32:39AM +0100, Mark Rutland wrote: > > > On Tue, Jun 30, 2020 at 06:37:20PM +0100, Will Deacon wrote: > > > > -#define read_barrier_depends() __asm__ __volatile__("mb": : :"memory") > > > > +#define __smp_load_acquire(p) \ > > > > +({ \ > > > > + __unqual_scalar_typeof(*p) ___p1 = \ > > > > + (*(volatile typeof(___p1) *)(p)); \ > > > > + compiletime_assert_atomic_type(*p); \ > > > > + ___p1; \ > > > > +}) > > > > > > Sorry if I'm being thick, but doesn't this need a barrier after the > > > volatile access to provide the acquire semantic? > > > > > > IIUC prior to this commit alpha would have used the asm-generic > > > __smp_load_acquire, i.e. > > > > > > | #ifndef __smp_load_acquire > > > | #define __smp_load_acquire(p) \ > > > | ({ \ > > > | __unqual_scalar_typeof(*p) ___p1 = READ_ONCE(*p); \ > > > | compiletime_assert_atomic_type(*p); \ > > > | __smp_mb(); \ > > > | (typeof(*p))___p1; \ > > > | }) > > > | #endif > > I also have a question that I didn't dare ask when the same > code came up before (I guess it's also what's in the kernel today): > > With the cast to 'typeof(*p)' at the end, doesn't that mean we > lose the effect of __unqual_scalar_typeof() again, so any "volatile" > pointer passed into __READ_ONCE_SCALAR() or > __smp_load_acquire() still leads to a volatile load of the original > variable, plus another volatile access to ___p1 after > spilling it to the stack as a non-volatile variable?
Not sure I follow you here, but I can confirm that what you're worried about doesn't happen for the usual case of a pointer-to-volatile scalar.
For example, ignoring dependency ordering:
unsigned long foo(volatile unsigned long *p) { return smp_load_acquire(p) + 1; }
Ends up looking like:
unsigned long ___p1 = *(const volatile unsigned long *)p; smp_mb(); (volatile unsigned long)___p1;
My understanding is that casting a non-pointer type to volatile doesn't do anything, so we're good.
On the other hand, you can still cause the stack reload if you use volatile pointers to volatile:
volatile unsigned long *bar(volatile unsigned long * volatile *ptr) { return READ_ONCE(*ptr); }
but this is pretty weird code, I think.
Will
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