arm: add non-assembly versions of pthread_spin_lock/pthread_spin_trylock

Sync pthread_spin_lock/pthread_spin_trylock with GNU libc as it avoids
any usage of deprecated SWP instruction. This allows to build uClibc-ng
for ARMv8 Cortex-A53 CPU in 32 Bit mode.

libpthread/nptl/sysdeps/arm/pthread_spin_lock.c

@@ -0,0 +1,66 @@
+/* pthread_spin_lock -- lock a spin lock.  Generic version.
+   Copyright (C) 2012-2016 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/>.  */
+
+#include <atomic.h>
+#include "pthreadP.h"
+
+/* A machine-specific version can define SPIN_LOCK_READS_BETWEEN_CMPXCHG
+  to the number of plain reads that it's optimal to spin on between uses
+  of atomic_compare_and_exchange_val_acq.  If spinning forever is optimal
+  then use -1.  If no plain reads here would ever be optimal, use 0.  */
+#define SPIN_LOCK_READS_BETWEEN_CMPXCHG 1000
+
+int
+pthread_spin_lock (pthread_spinlock_t *lock)
+{
+  /* atomic_exchange usually takes less instructions than
+     atomic_compare_and_exchange.  On the other hand,
+     atomic_compare_and_exchange potentially generates less bus traffic
+     when the lock is locked.
+     We assume that the first try mostly will be successful, and we use
+     atomic_exchange.  For the subsequent tries we use
+     atomic_compare_and_exchange.  */
+  if (atomic_exchange_acq (lock, 1) == 0)
+    return 0;
+
+  do
+    {
+      /* The lock is contended and we need to wait.  Going straight back
+	 to cmpxchg is not a good idea on many targets as that will force
+	 expensive memory synchronizations among processors and penalize other
+	 running threads.
+	 On the other hand, we do want to update memory state on the local core
+	 once in a while to avoid spinning indefinitely until some event that
+	 will happen to update local memory as a side-effect.  */
+      if (SPIN_LOCK_READS_BETWEEN_CMPXCHG >= 0)
+	{
+	  int wait = SPIN_LOCK_READS_BETWEEN_CMPXCHG;
+
+	  while (*lock != 0 && wait > 0)
+	    --wait;
+	}
+      else
+	{
+	  while (*lock != 0)
+	    ;
+	}
+    }
+  while (atomic_compare_and_exchange_val_acq (lock, 1, 0) != 0);
+
+  return 0;
+}

libpthread/nptl/sysdeps/arm/pthread_spin_trylock.S

@@ -1,33 +0,0 @@
-/* Copyright (C) 2005 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/>.  */
-
-#define _ERRNO_H 1
-#include <bits/errno.h>
-
-#include <sysdep.h>
-
-	.text
-	.align	4
-
-ENTRY (pthread_spin_trylock)
-	mov	r1, #1
-	swp	r2, r1, [r0]
-	teq	r2, #0
-	moveq	r0, #0
-	movne	r0, #EBUSY
-	PSEUDO_RET_NOERRNO
-END (pthread_spin_trylock)

libpthread/nptl/sysdeps/arm/pthread_spin_lock.S → libpthread/nptl/sysdeps/arm/pthread_spin_trylock.c

@@ -1,4 +1,5 @@
-/* Copyright (C) 2005 Free Software Foundation, Inc.
+/* pthread_spin_trylock -- trylock a spin lock.  Generic version.
+   Copyright (C) 2012-2016 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
@@ -15,16 +16,12 @@
    License along with the GNU C Library; if not, see
    <http://www.gnu.org/licenses/>.  */
 
-#include <sysdep.h>
+#include <errno.h>
+#include <atomic.h>
+#include "pthreadP.h"
 
-	.text
-	.align	4
-
-ENTRY (pthread_spin_lock)
-	mov	r1, #1
-1:	swp	r2, r1, [r0]
-	teq	r2, #0
-	bne	1b
-	mov	r0, #0
-	PSEUDO_RET_NOERRNO
-END (pthread_spin_lock)
+int
+pthread_spin_trylock (pthread_spinlock_t *lock)
+{
+  return atomic_exchange_acq (lock, 1) ? EBUSY : 0;
+}