Re: [PATCH] MIPS: Don't declare __current_thread_info globally

[Nathan Chancellor <natechancellor@gmail.com>]

On Wed, Jan 01, 2020 at 07:53:45PM -0500, Arvind Sankar wrote:
> On Wed, Jan 01, 2020 at 09:51:02PM +0100, Arnd Bergmann wrote:
> > On Wed, Jan 1, 2020 at 6:57 PM Paul Burton <paulburton@kernel.org> wrote:
> > > diff --git a/arch/mips/include/asm/thread_info.h b/arch/mips/include/asm/thread_info.h
> > > index 4993db40482c..aceefc3f9a1a 100644
> > > --- a/arch/mips/include/asm/thread_info.h
> > > +++ b/arch/mips/include/asm/thread_info.h
> > > @@ -50,10 +50,10 @@ struct thread_info {
> > >  }
> > >
> > >  /* How to get the thread information struct from C.  */
> > > -register struct thread_info *__current_thread_info __asm__("$28");
> > > -
> > >  static inline struct thread_info *current_thread_info(void)
> > >  {
> > > +       register struct thread_info *__current_thread_info __asm__("$28");
> > > +
> > >         return __current_thread_info;
> > >  }
> > 
> > This looks like a nice fix, but are you sure it doesn't allow the compiler to
> > reuse $28 for another purpose in the kernel under register pressure,
> > which would break current_thread_info()?
> > 
> > I see in the MIPS ABI document that $28 is preserved across function
> > calls, but I don't see any indication that a function is not allowed
> > to modify it and later restore the original content.
> > 
> >         Arnd
> 
> The compiler can already do that even with a global definition.
> 
> The doc since gcc 9 [1] says:
> 
> "Accesses to the variable may be optimized as usual and the register
> remains available for allocation and use in any computations, provided
> that observable values of the variable are not affected."
> 
> and
> 
> "Furthermore, since the register is not reserved exclusively for the
> variable, accessing it from handlers of asynchronous signals may observe
> unrelated temporary values residing in the register."
> 
> I'm not sure if this was a change in gcc 9 or simply the doc was wrong
> earlier.
> 
> Should there be a -ffixed-28 cflag for MIPS? alpha and hexagon seem to
> have that and they also keep current_thread_info in a register.
> 
> Also, commit fe92da0f355e9 ("MIPS: Changed current_thread_info() to an
> equivalent supported by both clang and GCC") moved this from local to
> global because local apparently didn't work on clang?
> 
> [1] https://gcc.gnu.org/onlinedocs/gcc-9.1.0/gcc/Global-Register-Variables.html

Yeah this patch appears to break booting malta_defconfig in QEMU when
built with clang; additionally, there are a TON of warnings about this
variable being uninitialized:

../arch/mips/include/asm/thread_info.h:57:9: warning: variable '__current_thread_info' is uninitialized when used here [-Wuninitialized]
        return __current_thread_info;
               ^~~~~~~~~~~~~~~~~~~~~
../arch/mips/include/asm/thread_info.h:55:52: note: initialize the variable '__current_thread_info' to silence this warning
        register struct thread_info *__current_thread_info __asm__("$28");
                                                          ^
                                                           = NULL
1 warning generated.

Seems like this is expected according to that previous commit? I
noticed there is another instance in arch/mips but it doesn't appear to
affect everything.

https://github.com/ClangBuiltLinux/linux/issues/606

Cheers,
Nathan