Re: [PATCH] LoongArch: Change __my_cpu_offset definition to avoid mis-optimization

[Huacai Chen]

Hi, Ruoyao,

On Wed, Mar 20, 2024 at 6:27 PM Xi Ruoyao <xry111@xxxxxxxxxxx> wrote:
>
> On Fri, 2024-03-15 at 10:45 +0800, Huacai Chen wrote:
> > From GCC commit 3f13154553f8546a ("df-scan: remove ad-hoc handling of
> > global regs in asms"), global registers will no longer be forced to add
> > to the def-use chain. Then current_thread_info(), current_stack_pointer
> > and __my_cpu_offset may be lifted out of the loop because they are no
> > longer treated as "volatile variables".
>
> Ooops...  I'm wondering why this issue has not blown up our systems
> before.  The referred GCC commit is far before LoongArch CPUs are taped.
>
> > This optimization is still correct for the current_thread_info() and
> > current_stack_pointer usages because they are associated to a thread.
> > However it is wrong for __my_cpu_offset because it is associated to a
> > CPU rather than a thread: if the thread migrates to a different CPU in
> > the loop, __my_cpu_offset should be changed.
> >
> > Change __my_cpu_offset definition to treat it as a "volatile variable",
> > in order to avoid such a mis-optimization.
> >
> > Cc: stable@xxxxxxxxxxxxxxx
>
> I suppose we should add Fixes: 5b0b14e550a0 ("LoongArch: Add
> atomic/locking header") here.
You are right, but since this patch is in loongarch-next for the pull
request, I don't want to change things.

Huacai

>
> > Reported-by: Xiaotian Wu <wuxiaotian@xxxxxxxxxxx>
> > Reported-by: Miao Wang <shankerwangmiao@xxxxxxxxx>
> > Signed-off-by: Xing Li <lixing@xxxxxxxxxxx>
> > Signed-off-by: Hongchen Zhang <zhanghongchen@xxxxxxxxxxx>
> > Signed-off-by: Rui Wang <wangrui@xxxxxxxxxxx>
> > Signed-off-by: Huacai Chen <chenhuacai@xxxxxxxxxxx>
> > ---
> >  arch/loongarch/include/asm/percpu.h | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/loongarch/include/asm/percpu.h b/arch/loongarch/include/asm/percpu.h
> > index 9b36ac003f89..03b98491d301 100644
> > --- a/arch/loongarch/include/asm/percpu.h
> > +++ b/arch/loongarch/include/asm/percpu.h
> > @@ -29,7 +29,12 @@ static inline void set_my_cpu_offset(unsigned long off)
> >       __my_cpu_offset = off;
> >       csr_write64(off, PERCPU_BASE_KS);
> >  }
> > -#define __my_cpu_offset __my_cpu_offset
> > +
> > +#define __my_cpu_offset                                      \
> > +({                                                   \
> > +     __asm__ __volatile__("":"+r"(__my_cpu_offset)); \
> > +     __my_cpu_offset;                                \
> > +})
> >
> >  #define PERCPU_OP(op, asm_op, c_op)                                  \
> >  static __always_inline unsigned long __percpu_##op(void *ptr,                \
>
> --
> Xi Ruoyao <xry111@xxxxxxxxxxx>
> School of Aerospace Science and Technology, Xidian University