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();
}
Trunk clang optimizes the comparison of the two pointers down
to "must be always true" even though they might not be if the
switcher() function has resulted in a change-of-thread:
https://godbolt.org/z/hd67zh6jW
The 'optnone' attribute (clang-specific) seems to be able
to suppress this attribute. The equivalent on gcc may
(or may not) be 'noipa'.
-- PMM
