On Thu, Dec 02, 2021 at 02:44:42PM +0000, Peter Maydell wrote:
> On Wed, 1 Dec 2021 at 17:19, Stefan Hajnoczi <stefa...@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
