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Date: Mon, 15 Oct 2012 21:10:18 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Ingo Molnar <mingo@...e.hu>,
Linus Torvalds <torvalds@...ux-foundation.org>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Peter Zijlstra <peterz@...radead.org>,
Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc: Ananth N Mavinakayanahalli <ananth@...ibm.com>,
Anton Arapov <anton@...hat.com>, linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] brw_mutex: big read-write mutex
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@...hat.com>
---
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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