[PATCH 1/2] brw_mutex: big read-write mutex

[Oleg Nesterov]

This patch adds the new sleeping lock, brw_mutex. Unlike rw_semaphore
it allows multiple writers too, just "read" and "write" are mutually
exclusive.

brw_start_read() and brw_end_read() are extremely cheap, they only do
this_cpu_inc(read_ctr) + atomic_read() if there are no waiting writers.

OTOH it is write-biased, any brw_start_write() blocks the new readers.
But "write" is slow, it does synchronize_sched() to serialize with
preempt_disable() in brw_start_read(), and wait_event(write_waitq) can
have a lot of extra wakeups before percpu-counter-sum becomes zero.

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
---
 include/linux/brw_mutex.h |   22 +++++++++++++++
 lib/Makefile              |    2 +-
 lib/brw_mutex.c           |   67 +++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 90 insertions(+), 1 deletions(-)
 create mode 100644 include/linux/brw_mutex.h
 create mode 100644 lib/brw_mutex.c

diff --git a/include/linux/brw_mutex.h b/include/linux/brw_mutex.h
new file mode 100644
index 0000000..16b8d5f
--- /dev/null
+++ b/include/linux/brw_mutex.h
@@ -0,0 +1,22 @@
+#ifndef _LINUX_BRW_MUTEX_H
+#define _LINUX_BRW_MUTEX_H
+
+#include <linux/percpu.h>
+#include <linux/wait.h>
+
+struct brw_mutex {
+	long __percpu		*read_ctr;
+	atomic_t		write_ctr;
+	wait_queue_head_t	read_waitq;
+	wait_queue_head_t	write_waitq;
+};
+
+extern int brw_mutex_init(struct brw_mutex *brw);
+
+extern void brw_start_read(struct brw_mutex *brw);
+extern void brw_end_read(struct brw_mutex *brw);
+
+extern void brw_start_write(struct brw_mutex *brw);
+extern void brw_end_write(struct brw_mutex *brw);
+
+#endif
diff --git a/lib/Makefile b/lib/Makefile
index 3128e35..18f2876 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -12,7 +12,7 @@ lib-y := ctype.o string.o vsprintf.o cmdline.o \
 	 idr.o int_sqrt.o extable.o \
 	 sha1.o md5.o irq_regs.o reciprocal_div.o argv_split.o \
 	 proportions.o flex_proportions.o prio_heap.o ratelimit.o show_mem.o \
-	 is_single_threaded.o plist.o decompress.o
+	 is_single_threaded.o plist.o decompress.o brw_mutex.o
 
 lib-$(CONFIG_MMU) += ioremap.o
 lib-$(CONFIG_SMP) += cpumask.o
diff --git a/lib/brw_mutex.c b/lib/brw_mutex.c
new file mode 100644
index 0000000..41984a6
--- /dev/null
+++ b/lib/brw_mutex.c
@@ -0,0 +1,67 @@
+#include <linux/brw_mutex.h>
+#include <linux/rcupdate.h>
+#include <linux/sched.h>
+
+int brw_mutex_init(struct brw_mutex *brw)
+{
+	atomic_set(&brw->write_ctr, 0);
+	init_waitqueue_head(&brw->read_waitq);
+	init_waitqueue_head(&brw->write_waitq);
+	brw->read_ctr = alloc_percpu(long);
+	return brw->read_ctr ? 0 : -ENOMEM;
+}
+
+void brw_start_read(struct brw_mutex *brw)
+{
+	for (;;) {
+		bool done = false;
+
+		preempt_disable();
+		if (likely(!atomic_read(&brw->write_ctr))) {
+			__this_cpu_inc(*brw->read_ctr);
+			done = true;
+		}
+		preempt_enable();
+
+		if (likely(done))
+			break;
+
+		__wait_event(brw->read_waitq, !atomic_read(&brw->write_ctr));
+	}
+}
+
+void brw_end_read(struct brw_mutex *brw)
+{
+	this_cpu_dec(*brw->read_ctr);
+
+	if (unlikely(atomic_read(&brw->write_ctr)))
+		wake_up_all(&brw->write_waitq);
+}
+
+static inline long brw_read_ctr(struct brw_mutex *brw)
+{
+	long sum = 0;
+	int cpu;
+
+	for_each_possible_cpu(cpu)
+		sum += per_cpu(*brw->read_ctr, cpu);
+
+	return sum;
+}
+
+void brw_start_write(struct brw_mutex *brw)
+{
+	atomic_inc(&brw->write_ctr);
+	synchronize_sched();
+	/*
+	 * Thereafter brw_*_read() must see write_ctr != 0,
+	 * and we should see the result of __this_cpu_inc().
+	 */
+	wait_event(brw->write_waitq, brw_read_ctr(brw) == 0);
+}
+
+void brw_end_write(struct brw_mutex *brw)
+{
+	if (atomic_dec_and_test(&brw->write_ctr))
+		wake_up_all(&brw->read_waitq);
+}
-- 
1.5.5.1

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