nptl: Add struct_rwlock.h

This patch adds a new generic __pthread_rwlock_arch_t definition meant
to be used by new ports.  Its layout mimics the current usage on some
64 bits ports and it allows some ports to use the generic definition.
The arch __pthread_rwlock_arch_t definition is moved from
pthreadtypes-arch.h to another arch-specific header (struct_rwlock.h).

Also the static intialization macro for pthread_rwlock_t is set to use
an arch defined on (__PTHREAD_RWLOCK_INITIALIZER) which simplifies its
implementation.

The default pthread_rwlock_t layout differs from current ports with:

  1. Internal layout is the same for 32 bits and 64 bits.

  2. Internal flag is an unsigned short so it should not required
     additional padding to align for word boundary (if it is the case
     for the ABI).

Checked with a build on affected abis.

Change-Id: I776a6a986c23199929d28a3dcd30272db21cd1d0

2 files changed, 75 insertions, 4 deletions

diff --git a/sysdeps/nptl/bits/struct_rwlock.h b/sysdeps/nptl/bits/struct_rwlock.h
new file mode 100644
index 0000000000..bb63028a46
--- /dev/null
+++ b/sysdeps/nptl/bits/struct_rwlock.h
@@ -0,0 +1,61 @@
+/* Default read-write lock implementation struct definitions.
+   Copyright (C) 2019 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#ifndef __RWLOCK_INTERNAL_H
+#define __RWLOCK_INTERNAL_H
+
+#include <bits/endian.h>
+
+/* Generic struct for both POSIX read-write lock.  New ports are expected
+   to use the default layout, however archictetures can redefine it to add
+   arch-specific extensions (such as lock-elision).  The struct have a size
+   of 32 bytes on both LP32 and LP64 architectures.  */
+
+struct __pthread_rwlock_arch_t
+{
+  unsigned int __readers;
+  unsigned int __writers;
+  unsigned int __wrphase_futex;
+  unsigned int __writers_futex;
+  unsigned int __pad3;
+  unsigned int __pad4;
+  /* FLAGS must stay at its position in the structure to maintain
+     binary compatibility.  */
+#if __BYTE_ORDER == __BIG_ENDIAN
+  unsigned char __pad1;
+  unsigned char __pad2;
+  unsigned char __shared;
+  unsigned char __flags;
+#else
+  unsigned char __flags;
+  unsigned char __shared;
+  unsigned char __pad1;
+  unsigned char __pad2;
+#endif
+  int __cur_writer;
+};
+
+#if __BYTE_ORDER == __BIG_ENDIAN
+# define __PTHREAD_RWLOCK_INITIALIZER(__flags) \
+  0, 0, 0, 0, 0, 0, 0, 0, 0, __flags, 0
+#else
+# define __PTHREAD_RWLOCK_INITIALIZER(__flags) \
+  0, 0, 0, 0, 0, 0, __flags, 0, 0, 0, 0
+#endif
+
+#endif
diff --git a/sysdeps/nptl/bits/thread-shared-types.h b/sysdeps/nptl/bits/thread-shared-types.h
index 0727fdec96..a409fa78a8 100644
--- a/sysdeps/nptl/bits/thread-shared-types.h
+++ b/sysdeps/nptl/bits/thread-shared-types.h
@@ -39,10 +39,7 @@
 
    Same idea but for the once locking primitive:
 
-   __ONCE_ALIGNMENT - for pthread_once_t/once_flag definition.
-
-   And finally the internal pthread_rwlock_t (struct __pthread_rwlock_arch_t)
-   must be defined.  */
+   __ONCE_ALIGNMENT - for pthread_once_t/once_flag definition.  */
 
 #include <bits/pthreadtypes-arch.h>
 
@@ -76,6 +73,19 @@ typedef struct __pthread_internal_slist
 
 #include <bits/struct_mutex.h>
 
+/* Arch-sepecific read-write lock definitions.  A generic implementation is
+   provided by struct_rwlock.h.  If required, an architecture can override it
+   by defining:
+
+   1. struct __pthread_rwlock_arch_t (used on pthread_rwlock_t definition).
+      It should contain at least the internal members defined in the
+      generic version.
+
+   2. The macro __PTHREAD_RWLOCK_INITIALIZER used for static initialization.
+      It should initialize the rwlock internal type.  */
+
+#include <bits/struct_rwlock.h>
+
 
 /* Common definition of pthread_cond_t. */