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SubjectRe: [patch 2.4.11-pre5] atomic_dec_and_lock() for alpha
On Mon, Oct 08, 2001 at 10:24:12AM -0700, Richard Henderson wrote:
> I am extremely uncomfortable with you returning out of the middle of
> an asm statement. What if the compiler decides to allocate a stack
> frame for some reason? You'll return without deallocating it.

Indeed.

> Please write the whole thing in assembly or avoid the early return.

OK. I prefer the latter - rewriting in assembly won't allow DEBUG_SPINLOCK
stuff in this function. OTOH, moving the return outside an asm statement
adds only one instruction - conditional branch that falls through in the
fast path.

Ivan.

--- 2.4.11p5/arch/alpha/lib/dec_and_lock.c Thu Jan 1 00:00:00 1970
+++ linux/arch/alpha/lib/dec_and_lock.c Tue Oct 9 14:15:01 2001
@@ -0,0 +1,37 @@
+/*
+ * arch/alpha/lib/dec_and_lock.c
+ *
+ * ll/sc version of atomic_dec_and_lock()
+ */
+
+#include <linux/compiler.h>
+#include <linux/spinlock.h>
+#include <asm/atomic.h>
+
+int atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
+{
+ long cnt;
+ __asm__ __volatile__(
+ "1: ldl_l %0,%1\n"
+ " subl %0,1,%0\n"
+ " beq %0,2f\n"
+ " stl_c %0,%1\n"
+ " beq %0,3f\n"
+ " mb\n"
+ "2:\n"
+ ".subsection 2\n"
+ "3: br 1b\n"
+ ".previous"
+ :"=&r" (cnt), "=m" (atomic->counter)
+ :"m" (atomic->counter) : "memory");
+
+ if (likely(cnt))
+ return 0;
+
+ /* Slow path */
+ spin_lock(lock);
+ if (atomic_dec_and_test(atomic))
+ return 1;
+ spin_unlock(lock);
+ return 0;
+}
--- 2.4.11p5/arch/alpha/lib/Makefile Wed Jun 20 22:10:27 2001
+++ linux/arch/alpha/lib/Makefile Fri Oct 5 17:37:20 2001
@@ -49,6 +49,10 @@ OBJS = __divqu.o __remqu.o __divlu.o __r
fpreg.o \
callback_srm.o srm_puts.o srm_printk.o

+ifeq ($(CONFIG_SMP),y)
+ OBJS += dec_and_lock.o
+endif
+
lib.a: $(OBJS)
$(AR) rcs lib.a $(OBJS)

--- 2.4.11p5/arch/alpha/kernel/alpha_ksyms.c Fri Sep 14 02:21:32 2001
+++ linux/arch/alpha/kernel/alpha_ksyms.c Fri Oct 5 19:56:39 2001
@@ -215,6 +215,7 @@ EXPORT_SYMBOL(__global_cli);
EXPORT_SYMBOL(__global_sti);
EXPORT_SYMBOL(__global_save_flags);
EXPORT_SYMBOL(__global_restore_flags);
+EXPORT_SYMBOL(atomic_dec_and_lock);
#if DEBUG_SPINLOCK
EXPORT_SYMBOL(spin_unlock);
EXPORT_SYMBOL(debug_spin_lock);
--- 2.4.11p5/arch/alpha/config.in Fri Oct 5 17:27:50 2001
+++ linux/arch/alpha/config.in Fri Oct 5 17:37:20 2001
@@ -217,6 +217,10 @@ then
bool 'Symmetric multi-processing support' CONFIG_SMP
fi

+if [ "$CONFIG_SMP" = "y" ]; then
+ define_bool CONFIG_HAVE_DEC_LOCK y
+fi
+
if [ "$CONFIG_EXPERIMENTAL" = "y" ]; then
bool 'Discontiguous Memory Support' CONFIG_DISCONTIGMEM
if [ "$CONFIG_DISCONTIGMEM" = "y" ]; then
-
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