Re: [PATCH linux-2.6 01/04] brsem: implement big reader semaphore

[Nick Piggin <nickpiggin@yahoo.com.au>]

Nick Piggin wrote:

> What would be wrong with an array of NR_CPUS rwsems? The only
> tiny trick you would have to do AFAIKS is have up_read remember
> what rwsem down_read took, but that could be returned from
> down_read as a token.

Here's something to start with. Add initialisers / cacheline
alignment / percpu_alloc to taste.

For bonus points I guess you could change the implementation
to just a single waitqueue mechanism with NR_CPUs counters.

In fact, it probably needs quite a bit of work before it is
mainline-worthy, but as a proof of concept - do you see
anything wrong with it?

Nick

--
SUSE Labs, Novell Inc.

Index: linux-2.6/include/linux/brsem.h
===================================================================
--- /dev/null
+++ linux-2.6/include/linux/brsem.h
@@ -0,0 +1,18 @@
+#ifndef __BRSEM_H
+#define __BRSEM_H
+
+#include <linux/rwsem.h>
+struct brsem {
+	struct rw_semaphore cpu_sem[NR_CPUS];
+};
+
+#define BRSEM_READ_TRYLOCK_FAILED	-1
+typedef int brsem_read_t;
+
+brsem_read_t brsem_down_read(struct brsem *);
+brsem_read_t brsem_down_read_trylock(struct brsem *);
+void brsem_up_read(struct brsem *, brsem_read_t);
+void brsem_down_write(struct brsem *);
+void brsem_up_write(struct brsem *);
+
+#endif
Index: linux-2.6/lib/Makefile
===================================================================
--- linux-2.6.orig/lib/Makefile
+++ linux-2.6/lib/Makefile
@@ -5,7 +5,7 @@
 lib-y := errno.o ctype.o string.o vsprintf.o cmdline.o \
 	 bust_spinlocks.o rbtree.o radix-tree.o dump_stack.o \
 	 idr.o div64.o int_sqrt.o bitmap.o extable.o prio_tree.o \
-	 sha1.o
+	 sha1.o brsem.o
 
 lib-y	+= kobject.o kref.o kobject_uevent.o klist.o
 
Index: linux-2.6/lib/brsem.c
===================================================================
--- /dev/null
+++ linux-2.6/lib/brsem.c
@@ -0,0 +1,38 @@
+#include <linux/brsem.h>
+#include <linux/rwsem.h>
+#include <linux/smp.h>
+
+brsem_read_t brsem_down_read(struct brsem *brsem)
+{
+	brsem_read_t ret = smp_processor_id();
+	down_read(&brsem->cpu_sem[ret]);
+	return ret;
+}
+
+brsem_read_t brsem_down_read_trylock(struct brsem *brsem)
+{
+	brsem_read_t ret = smp_processor_id();
+	if (!down_read_trylock(&brsem->cpu_sem[ret]))
+		return BRSEM_READ_TRYLOCK_FAILED;
+	return ret;
+}
+
+void brsem_up_read(struct brsem *brsem, brsem_read_t token)
+{
+	up_read(&brsem->cpu_sem[token]);
+}
+
+void brsem_down_write(struct brsem *brsem)
+{
+	int i;
+	for (i = 0; i < NR_CPUS; i++)
+		down_write(&brsem->cpu_sem[i]);
+}
+
+void brsem_up_write(struct brsem *brsem)
+{
+	int i;
+	for (i = 0; i < NR_CPUS; i++)
+		up_write(&brsem->cpu_sem[i]);
+}
+