Re: [RFC v2 1/4] tls: add macros for coroutine-safe TLS variables

[Serge Guelton]

On Thu, Dec 02, 2021 at 02:44:42PM +0000, Peter Maydell wrote:
> On Wed, 1 Dec 2021 at 17:19, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> >
> > Compiler optimizations can cache TLS values across coroutine yield
> > points, resulting in stale values from the previous thread when a
> > coroutine is re-entered by a new thread.
> >
> > Serge Guelton developed an __attribute__((noinline)) wrapper and tested
> > it with clang and gcc. I formatted his idea according to QEMU's coding
> > style and wrote documentation.
> 
> > +#ifdef QEMU_CO_TLS_ADDR
> > +#define QEMU_DEFINE_STATIC_CO_TLS(type, var)                    \
> > +    __thread type co_tls_##var;  \
> > +    static inline type get_##var(void)                          \
> > +    { type *p; QEMU_CO_TLS_ADDR(p, co_tls_##var); return *p; }  \
> > +    static inline void set_##var(type v)                        \
> > +    { type *p; QEMU_CO_TLS_ADDR(p, co_tls_##var); *p = v; }     \
> > +    static inline type *get_ptr_##var(void)                     \
> > +    { type *p; QEMU_CO_TLS_ADDR(p, co_tls_##var); return p; }
> > +#else
> > +#define QEMU_DEFINE_STATIC_CO_TLS(type, var)                    \
> > +    static __thread type co_tls_##var;                          \
> > +    static __attribute__((noinline, unused)) type get_##var(void)       \
> > +    { return co_tls_##var; }                                    \
> > +    static __attribute__((noinline, unused)) void set_##var(type v)     \
> > +    { co_tls_##var = v; }                                       \
> > +    static __attribute__((noinline, unused)) type *get_ptr_##var(void)  \
> > +    { return &co_tls_##var; }
> > +#endif
> 
> My compiler-developer colleagues present the following case where
> 'noinline' is not sufficient for the compiler to definitely
> use different values of the address-of-the-TLS-var across an
> intervening function call:
> 
>   __thread int i;
> 
>   __attribute__((noinline)) long get_ptr_i()
>   {
>     return (long)&i;
>   }
> 
>   void switcher();
> 
>   int g()
>   {
>     long a = get_ptr_i();
>     switcher();
>     return a == get_ptr_i();
>   }

You can also force an extra mov through `volatile` as in
https://godbolt.org/z/hWvdb7o9G