Messages in this thread | | | From | Uros Bizjak <> | Date | Wed, 18 Oct 2023 20:26:55 +0200 | Subject | Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu_ptr() |
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On Wed, Oct 18, 2023 at 8:16 PM Linus Torvalds <torvalds@linux-foundation.org> wrote: > > On Wed, 18 Oct 2023 at 11:08, Uros Bizjak <ubizjak@gmail.com> wrote: > > > > But loads from non-const memory work like the above. > > Yes, I'm certainly ok with the move to use plain loads from __seg_gs > for the percpu accesses. If they didn't honor the memory clobber, we > could never use it at all. > > I was just saying that the 'const' alias trick isn't useful for > anything else than 'current', because everything else needs to at > least honor our existing barriers.
FYI, smp_processor_id() is implemented as:
#define __smp_processor_id() __this_cpu_read(pcpu_hot.cpu_number)
where __this_* forces volatile access which disables CSE.
*If* the variable is really stable, then it should use __raw_cpu_read. Both, __raw_* and __this_* were recently (tip/percpu branch) implemented for SEG_SUPPORT as:
#define __raw_cpu_read(qual, pcp) \ ({ \ *(qual __my_cpu_type(pcp) *)__my_cpu_ptr(&(pcp)); \ })
where "qual" can be volatile. To enable smp_processor_id() optimization, it just needs to be moved from __this to __raw accessor.
> (And yes, there's the other user of this_cpu_read_stable() - > 'top_of_stack', but as mentioned that doesn't really matter).
Uros.
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