[PATCH 2/2] x86_64: Use __seg_tls for thread access when available

[Florian Weimer]

On 10/08/2015 05:58 AM, Richard Henderson wrote:
> The change in ARCH_FORK is to force conversion to generic address
> space before casting to an integral type; otherwise what we get is
> the unhelpful %fs-based address of tid.
> 
> The change in THREAD_ATOMIC_CMPXCHG_VAL is to avoid __typeof copying
> the address space from descr, leading to an error: automatic variable
> with non-default address space.

__SEG_TLS comes from the GCC patches.  It needs to be documented
somewhere, see my response on gcc-patches.

>  sysdeps/unix/sysv/linux/x86_64/arch-fork.h |  2 +-
>  sysdeps/x86_64/nptl/tls.h                  | 44 ++++++++++++++++++++++--------
>  2 files changed, 34 insertions(+), 12 deletions(-)

Changelog is missing.

>  /* Install new dtv for current thread.  */
>  # define INSTALL_NEW_DTV(dtvp) \
> -  ({ struct pthread *__pd;						      \
> +  ({ pthread_self_t *__pd = THREAD_SELF;				      \
>       THREAD_SETMEM (__pd, header.dtv, (dtvp)); })

Does this change code in the !__SEG_TLS case?  (THREAD_SETMEM does not
evaluate the __pd argument, which is why this worked before.)

>  /* Atomic compare and exchange on TLS, returning old value.  */
>  # define THREAD_ATOMIC_CMPXCHG_VAL(descr, member, newval, oldval) \
> -  ({ __typeof (descr->member) __ret;					      \
> +  ({ __typeof (((struct pthread *)0)->member) __ret;			      \
>       __typeof (oldval) __old = (oldval);				      \
>       if (sizeof (descr->member) == 4)					      \
>         asm volatile (LOCK_PREFIX "cmpxchgl %2, %%fs:%P3"		      \

The sizeofs should probably refer to __ret instead of descr->member for
consistency.

Florian