2 files changed, 91 insertions, 1 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index e60bbd0225d5..24e1229cdfe0 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -10,6 +10,7 @@
 #include <linux/ioctl.h>
 #include <linux/blk_types.h>
 #include <linux/types.h>
+#include <linux/percpu-rwsem.h>
 
 /*
  * It's silly to have NR_OPEN bigger than NR_FILE, but you can change
@@ -726,7 +727,7 @@ struct block_device {
 	/* Mutex for freeze */
 	struct mutex		bd_fsfreeze_mutex;
 	/* A semaphore that prevents I/O while block size is being changed */
-	struct rw_semaphore	bd_block_size_semaphore;
+	struct percpu_rw_semaphore	bd_block_size_semaphore;
 };
 
 /*
diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
new file mode 100644
index 000000000000..cf80f7e5277f
--- /dev/null
+++ b/include/linux/percpu-rwsem.h
@@ -0,0 +1,89 @@
+#ifndef _LINUX_PERCPU_RWSEM_H
+#define _LINUX_PERCPU_RWSEM_H
+
+#include <linux/mutex.h>
+#include <linux/percpu.h>
+#include <linux/rcupdate.h>
+#include <linux/delay.h>
+
+struct percpu_rw_semaphore {
+	unsigned __percpu *counters;
+	bool locked;
+	struct mutex mtx;
+};
+
+static inline void percpu_down_read(struct percpu_rw_semaphore *p)
+{
+	rcu_read_lock();
+	if (unlikely(p->locked)) {
+		rcu_read_unlock();
+		mutex_lock(&p->mtx);
+		this_cpu_inc(*p->counters);
+		mutex_unlock(&p->mtx);
+		return;
+	}
+	this_cpu_inc(*p->counters);
+	rcu_read_unlock();
+}
+
+static inline void percpu_up_read(struct percpu_rw_semaphore *p)
+{
+	/*
+	 * On X86, write operation in this_cpu_dec serves as a memory unlock
+	 * barrier (i.e. memory accesses may be moved before the write, but
+	 * no memory accesses are moved past the write).
+	 * On other architectures this may not be the case, so we need smp_mb()
+	 * there.
+	 */
+#if defined(CONFIG_X86) && (!defined(CONFIG_X86_PPRO_FENCE) && !defined(CONFIG_X86_OOSTORE))
+	barrier();
+#else
+	smp_mb();
+#endif
+	this_cpu_dec(*p->counters);
+}
+
+static inline unsigned __percpu_count(unsigned __percpu *counters)
+{
+	unsigned total = 0;
+	int cpu;
+
+	for_each_possible_cpu(cpu)
+		total += ACCESS_ONCE(*per_cpu_ptr(counters, cpu));
+
+	return total;
+}
+
+static inline void percpu_down_write(struct percpu_rw_semaphore *p)
+{
+	mutex_lock(&p->mtx);
+	p->locked = true;
+	synchronize_rcu();
+	while (__percpu_count(p->counters))
+		msleep(1);
+	smp_rmb(); /* paired with smp_mb() in percpu_sem_up_read() */
+}
+
+static inline void percpu_up_write(struct percpu_rw_semaphore *p)
+{
+	p->locked = false;
+	mutex_unlock(&p->mtx);
+}
+
+static inline int percpu_init_rwsem(struct percpu_rw_semaphore *p)
+{
+	p->counters = alloc_percpu(unsigned);
+	if (unlikely(!p->counters))
+		return -ENOMEM;
+	p->locked = false;
+	mutex_init(&p->mtx);
+	return 0;
+}
+
+static inline void percpu_free_rwsem(struct percpu_rw_semaphore *p)
+{
+	free_percpu(p->counters);
+	p->counters = NULL; /* catch use after free bugs */
+}
+
+#endif