Re: [PATCH v2 13/14] elf: Remove lazy tlsdesc relocation related code

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 13/04/2021 05:21, Szabolcs Nagy via Libc-alpha wrote:
> Remove generic tlsdesc code related to lazy tlsdesc processing since
> lazy tlsdesc relocation is no longer supported.  This includes removing
> GL(dl_load_lock) from _dl_make_tlsdesc_dynamic which is only called at
> load time when that lock is already held.
> 
> Added a documentation comment too.

LGTM, thanks.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

> ---
>  elf/tlsdeschtab.h         | 53 +++++----------------------------------
>  sysdeps/aarch64/tlsdesc.c |  1 -
>  sysdeps/arm/tlsdesc.c     |  1 -
>  sysdeps/i386/tlsdesc.c    |  1 -
>  sysdeps/x86_64/tlsdesc.c  |  1 -
>  5 files changed, 6 insertions(+), 51 deletions(-)
> 
> diff --git a/elf/tlsdeschtab.h b/elf/tlsdeschtab.h
> index 21695fd1e9..85bd0415e1 100644
> --- a/elf/tlsdeschtab.h
> +++ b/elf/tlsdeschtab.h
> @@ -78,6 +78,10 @@ map_generation (struct link_map *map)
>    return GL(dl_tls_generation) + 1;
>  }
>  
> +/* Returns the data pointer for a given map and tls offset that is used
> +   to fill in one of the GOT entries referenced by a TLSDESC relocation
> +   when using dynamic TLS.  This requires allocation, returns NULL on
> +   allocation failure.  */
>  void *
>  _dl_make_tlsdesc_dynamic (struct link_map *map, size_t ti_offset)
>  {
> @@ -85,18 +89,12 @@ _dl_make_tlsdesc_dynamic (struct link_map *map, size_t ti_offset)
>    void **entry;
>    struct tlsdesc_dynamic_arg *td, test;
>  
> -  /* FIXME: We could use a per-map lock here, but is it worth it?  */
> -  __rtld_lock_lock_recursive (GL(dl_load_lock));
> -
>    ht = map->l_mach.tlsdesc_table;
>    if (! ht)
>      {
>        ht = htab_create ();
>        if (! ht)
> -	{
> -	  __rtld_lock_unlock_recursive (GL(dl_load_lock));
> -	  return 0;
> -	}
> +	return 0;
>        map->l_mach.tlsdesc_table = ht;
>      }
>  

Ok.

> @@ -104,15 +102,11 @@ _dl_make_tlsdesc_dynamic (struct link_map *map, size_t ti_offset)
>    test.tlsinfo.ti_offset = ti_offset;
>    entry = htab_find_slot (ht, &test, 1, hash_tlsdesc, eq_tlsdesc);
>    if (! entry)
> -    {
> -      __rtld_lock_unlock_recursive (GL(dl_load_lock));
> -      return 0;
> -    }
> +    return 0;
>  
>    if (*entry)
>      {
>        td = *entry;
> -      __rtld_lock_unlock_recursive (GL(dl_load_lock));
>        return td;
>      }
>  
Ok.

> @@ -122,44 +116,9 @@ _dl_make_tlsdesc_dynamic (struct link_map *map, size_t ti_offset)
>       thread.  */
>    td->gen_count = map_generation (map);
>    td->tlsinfo = test.tlsinfo;
> -
> -  __rtld_lock_unlock_recursive (GL(dl_load_lock));
>    return td;
>  }
>  
>  # endif /* SHARED */
>  
> -/* The idea of the following two functions is to stop multiple threads
> -   from attempting to resolve the same TLS descriptor without busy
> -   waiting.  Ideally, we should be able to release the lock right
> -   after changing td->entry, and then using say a condition variable
> -   or a futex wake to wake up any waiting threads, but let's try to
> -   avoid introducing such dependencies.  */
> -
> -static int
> -__attribute__ ((unused))
> -_dl_tlsdesc_resolve_early_return_p (struct tlsdesc volatile *td, void *caller)
> -{
> -  if (caller != atomic_load_relaxed (&td->entry))
> -    return 1;
> -
> -  __rtld_lock_lock_recursive (GL(dl_load_lock));
> -  if (caller != atomic_load_relaxed (&td->entry))
> -    {
> -      __rtld_lock_unlock_recursive (GL(dl_load_lock));
> -      return 1;
> -    }
> -
> -  atomic_store_relaxed (&td->entry, _dl_tlsdesc_resolve_hold);
> -
> -  return 0;
> -}
> -
> -static void
> -__attribute__ ((unused))
> -_dl_tlsdesc_wake_up_held_fixups (void)
> -{
> -  __rtld_lock_unlock_recursive (GL(dl_load_lock));
> -}
> -
>  #endif

Ok.

> diff --git a/sysdeps/aarch64/tlsdesc.c b/sysdeps/aarch64/tlsdesc.c
> index 9b6a8b0ec4..3c1bcd8fe6 100644
> --- a/sysdeps/aarch64/tlsdesc.c
> +++ b/sysdeps/aarch64/tlsdesc.c
> @@ -22,7 +22,6 @@
>  #include <tls.h>
>  #include <dl-tlsdesc.h>
>  #include <dl-unmap-segments.h>
> -#define _dl_tlsdesc_resolve_hold 0
>  #include <tlsdeschtab.h>
>  
>  /* Unmap the dynamic object, but also release its TLS descriptor table

Ok.

> diff --git a/sysdeps/arm/tlsdesc.c b/sysdeps/arm/tlsdesc.c
> index 2f70adef5c..b3d1735af4 100644
> --- a/sysdeps/arm/tlsdesc.c
> +++ b/sysdeps/arm/tlsdesc.c
> @@ -20,7 +20,6 @@
>  #include <tls.h>
>  #include <dl-tlsdesc.h>
>  #include <dl-unmap-segments.h>
> -#define _dl_tlsdesc_resolve_hold 0
>  #include <tlsdeschtab.h>
>  
>  /* Unmap the dynamic object, but also release its TLS descriptor table

Ok.

> diff --git a/sysdeps/i386/tlsdesc.c b/sysdeps/i386/tlsdesc.c
> index 436a21f66b..364c7769ce 100644
> --- a/sysdeps/i386/tlsdesc.c
> +++ b/sysdeps/i386/tlsdesc.c
> @@ -20,7 +20,6 @@
>  #include <tls.h>
>  #include <dl-tlsdesc.h>
>  #include <dl-unmap-segments.h>
> -#define _dl_tlsdesc_resolve_hold 0
>  #include <tlsdeschtab.h>
>  
>  /* Unmap the dynamic object, but also release its TLS descriptor table

Ok.

> diff --git a/sysdeps/x86_64/tlsdesc.c b/sysdeps/x86_64/tlsdesc.c
> index ecf864d6ee..3bb80517a3 100644
> --- a/sysdeps/x86_64/tlsdesc.c
> +++ b/sysdeps/x86_64/tlsdesc.c
> @@ -20,7 +20,6 @@
>  #include <tls.h>
>  #include <dl-tlsdesc.h>
>  #include <dl-unmap-segments.h>
> -#define _dl_tlsdesc_resolve_hold 0
>  #include <tlsdeschtab.h>
>  
>  /* Unmap the dynamic object, but also release its TLS descriptor table
> 

Ok.