change sb_writers to use percpu_rw_semaphore

We can remove everything from struct sb_writers except frozen
and add the array of percpu_rw_semaphore's instead.

This patch doesn't remove sb_writers->wait_unfrozen yet, we keep
it for get_super_thawed(). We will probably remove it later.

This change tries to address the following problems:

	- Firstly, __sb_start_write() looks simply buggy. It does
	  __sb_end_write() if it sees ->frozen, but if it migrates
	  to another CPU before percpu_counter_dec(), sb_wait_write()
	  can wrongly succeed if there is another task which holds
	  the same "semaphore": sb_wait_write() can miss the result
	  of the previous percpu_counter_inc() but see the result
	  of this percpu_counter_dec().

	- As Dave Hansen reports, it is suboptimal. The trivial
	  microbenchmark that writes to a tmpfs file in a loop runs
	  12% faster if we change this code to rely on RCU and kill
	  the memory barriers.

	- This code doesn't look simple. It would be better to rely
	  on the generic locking code.

	  According to Dave, this change adds the same performance
	  improvement.

Note: with this change both freeze_super() and thaw_super() will do
synchronize_sched_expedited() 3 times. This is just ugly. But:

	- This will be "fixed" by the rcu_sync changes we are going
	  to merge. After that freeze_super()->percpu_down_write()
	  will use synchronize_sched(), and thaw_super() won't use
	  synchronize() at all.

	  This doesn't need any changes in fs/super.c.

	- Once we merge rcu_sync changes, we can also change super.c
	  so that all wb_write->rw_sem's will share the single ->rss
	  in struct sb_writes, then freeze_super() will need only one
	  synchronize_sched().
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Jan Kara <jack@suse.com>

fs/super.c

@@ -142,7 +142,7 @@ static void destroy_super_work(struct work_struct *work)
 	int i;
 
 	for (i = 0; i < SB_FREEZE_LEVELS; i++)
-		percpu_counter_destroy(&s->s_writers.counter[i]);
+		percpu_free_rwsem(&s->s_writers.rw_sem[i]);
 	kfree(s);
 }
 
@@ -193,13 +193,11 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags)
 		goto fail;
 
 	for (i = 0; i < SB_FREEZE_LEVELS; i++) {
-		if (percpu_counter_init(&s->s_writers.counter[i], 0,
-					GFP_KERNEL) < 0)
+		if (__percpu_init_rwsem(&s->s_writers.rw_sem[i],
+					sb_writers_name[i],
+					&type->s_writers_key[i]))
 			goto fail;
-		lockdep_init_map(&s->s_writers.lock_map[i], sb_writers_name[i],
-				 &type->s_writers_key[i], 0);
 	}
-	init_waitqueue_head(&s->s_writers.wait);
 	init_waitqueue_head(&s->s_writers.wait_unfrozen);
 	s->s_bdi = &noop_backing_dev_info;
 	s->s_flags = flags;
@@ -1161,47 +1159,10 @@ mount_fs(struct file_system_type *type, int flags, const char *name, void *data)
  */
 void __sb_end_write(struct super_block *sb, int level)
 {
-	percpu_counter_dec(&sb->s_writers.counter[level-1]);
-	/*
-	 * Make sure s_writers are updated before we wake up waiters in
-	 * freeze_super().
-	 */
-	smp_mb();
-	if (waitqueue_active(&sb->s_writers.wait))
-		wake_up(&sb->s_writers.wait);
-	rwsem_release(&sb->s_writers.lock_map[level-1], 1, _RET_IP_);
+	percpu_up_read(sb->s_writers.rw_sem + level-1);
 }
 EXPORT_SYMBOL(__sb_end_write);
 
-static int do_sb_start_write(struct super_block *sb, int level, bool wait,
-				unsigned long ip)
-{
-	if (wait)
-		rwsem_acquire_read(&sb->s_writers.lock_map[level-1], 0, 0, ip);
-retry:
-	if (unlikely(sb->s_writers.frozen >= level)) {
-		if (!wait)
-			return 0;
-		wait_event(sb->s_writers.wait_unfrozen,
-			   sb->s_writers.frozen < level);
-	}
-
-	percpu_counter_inc(&sb->s_writers.counter[level-1]);
-	/*
-	 * Make sure counter is updated before we check for frozen.
-	 * freeze_super() first sets frozen and then checks the counter.
-	 */
-	smp_mb();
-	if (unlikely(sb->s_writers.frozen >= level)) {
-		__sb_end_write(sb, level);
-		goto retry;
-	}
-
-	if (!wait)
-		rwsem_acquire_read(&sb->s_writers.lock_map[level-1], 0, 1, ip);
-	return 1;
-}
-
 /*
  * This is an internal function, please use sb_start_{write,pagefault,intwrite}
  * instead.
@@ -1209,7 +1170,7 @@ static int do_sb_start_write(struct super_block *sb, int level, bool wait,
 int __sb_start_write(struct super_block *sb, int level, bool wait)
 {
 	bool force_trylock = false;
-	int ret;
+	int ret = 1;
 
 #ifdef CONFIG_LOCKDEP
 	/*
@@ -1225,13 +1186,17 @@ int __sb_start_write(struct super_block *sb, int level, bool wait)
 		int i;
 
 		for (i = 0; i < level - 1; i++)
-			if (lock_is_held(&sb->s_writers.lock_map[i])) {
+			if (percpu_rwsem_is_held(sb->s_writers.rw_sem + i)) {
 				force_trylock = true;
 				break;
 			}
 	}
 #endif
-	ret = do_sb_start_write(sb, level, wait && !force_trylock, _RET_IP_);
+	if (wait && !force_trylock)
+		percpu_down_read(sb->s_writers.rw_sem + level-1);
+	else
+		ret = percpu_down_read_trylock(sb->s_writers.rw_sem + level-1);
+
 	WARN_ON(force_trylock & !ret);
 	return ret;
 }
@@ -1243,15 +1208,11 @@ EXPORT_SYMBOL(__sb_start_write);
  * @level: type of writers we wait for (normal vs page fault)
  *
  * This function waits until there are no writers of given type to given file
- * system. Caller of this function should make sure there can be no new writers
- * of type @level before calling this function. Otherwise this function can
- * livelock.
+ * system.
  */
 static void sb_wait_write(struct super_block *sb, int level)
 {
-	s64 writers;
-
-	rwsem_acquire(&sb->s_writers.lock_map[level-1], 0, 0, _THIS_IP_);
+	percpu_down_write(sb->s_writers.rw_sem + level-1);
 	/*
 	 * We are going to return to userspace and forget about this lock, the
 	 * ownership goes to the caller of thaw_super() which does unlock.
@@ -1262,24 +1223,18 @@ static void sb_wait_write(struct super_block *sb, int level)
 	 * this leads to lockdep false-positives, so currently we do the early
 	 * release right after acquire.
 	 */
-	rwsem_release(&sb->s_writers.lock_map[level-1], 1, _THIS_IP_);
-
-	do {
-		DEFINE_WAIT(wait);
+	percpu_rwsem_release(sb->s_writers.rw_sem + level-1, 0, _THIS_IP_);
+}
 
-		/*
-		 * We use a barrier in prepare_to_wait() to separate setting
-		 * of frozen and checking of the counter
-		 */
-		prepare_to_wait(&sb->s_writers.wait, &wait,
-				TASK_UNINTERRUPTIBLE);
+static void sb_freeze_unlock(struct super_block *sb)
+{
+	int level;
 
-		writers = percpu_counter_sum(&sb->s_writers.counter[level-1]);
-		if (writers)
-			schedule();
+	for (level = 0; level < SB_FREEZE_LEVELS; ++level)
+		percpu_rwsem_acquire(sb->s_writers.rw_sem + level, 0, _THIS_IP_);
 
-		finish_wait(&sb->s_writers.wait, &wait);
-	} while (writers);
+	for (level = SB_FREEZE_LEVELS - 1; level >= 0; level--)
+		percpu_up_write(sb->s_writers.rw_sem + level);
 }
 
 /**
@@ -1338,20 +1293,14 @@ int freeze_super(struct super_block *sb)
 		return 0;
 	}
 
-	/* From now on, no new normal writers can start */
 	sb->s_writers.frozen = SB_FREEZE_WRITE;
-	smp_wmb();
-
 	/* Release s_umount to preserve sb_start_write -> s_umount ordering */
 	up_write(&sb->s_umount);
-
 	sb_wait_write(sb, SB_FREEZE_WRITE);
+	down_write(&sb->s_umount);
 
 	/* Now we go and block page faults... */
-	down_write(&sb->s_umount);
 	sb->s_writers.frozen = SB_FREEZE_PAGEFAULT;
-	smp_wmb();
-
 	sb_wait_write(sb, SB_FREEZE_PAGEFAULT);
 
 	/* All writers are done so after syncing there won't be dirty data */
@@ -1359,7 +1308,6 @@ int freeze_super(struct super_block *sb)
 
 	/* Now wait for internal filesystem counter */
 	sb->s_writers.frozen = SB_FREEZE_FS;
-	smp_wmb();
 	sb_wait_write(sb, SB_FREEZE_FS);
 
 	if (sb->s_op->freeze_fs) {
@@ -1368,7 +1316,7 @@ int freeze_super(struct super_block *sb)
 			printk(KERN_ERR
 				"VFS:Filesystem freeze failed\n");
 			sb->s_writers.frozen = SB_UNFROZEN;
-			smp_wmb();
+			sb_freeze_unlock(sb);
 			wake_up(&sb->s_writers.wait_unfrozen);
 			deactivate_locked_super(sb);
 			return ret;
@@ -1400,8 +1348,10 @@ int thaw_super(struct super_block *sb)
 		return -EINVAL;
 	}
 
-	if (sb->s_flags & MS_RDONLY)
+	if (sb->s_flags & MS_RDONLY) {
+		sb->s_writers.frozen = SB_UNFROZEN;
 		goto out;
+	}
 
 	if (sb->s_op->unfreeze_fs) {
 		error = sb->s_op->unfreeze_fs(sb);
@@ -1413,12 +1363,11 @@ int thaw_super(struct super_block *sb)
 		}
 	}
 
-out:
 	sb->s_writers.frozen = SB_UNFROZEN;
-	smp_wmb();
+	sb_freeze_unlock(sb);
+out:
 	wake_up(&sb->s_writers.wait_unfrozen);
 	deactivate_locked_super(sb);
-
 	return 0;
 }
 EXPORT_SYMBOL(thaw_super);

include/linux/fs.h

 #ifndef _LINUX_FS_H
 #define _LINUX_FS_H
 
-
 #include <linux/linkage.h>
 #include <linux/wait.h>
 #include <linux/kdev_t.h>
@@ -31,6 +30,7 @@
 #include <linux/percpu-rwsem.h>
 #include <linux/blk_types.h>
 #include <linux/workqueue.h>
+#include <linux/percpu-rwsem.h>
 
 #include <asm/byteorder.h>
 #include <uapi/linux/fs.h>
@@ -1275,16 +1275,9 @@ enum {
 #define SB_FREEZE_LEVELS (SB_FREEZE_COMPLETE - 1)
 
 struct sb_writers {
-	/* Counters for counting writers at each level */
-	struct percpu_counter	counter[SB_FREEZE_LEVELS];
-	wait_queue_head_t	wait;		/* queue for waiting for
-						   writers / faults to finish */
-	int			frozen;		/* Is sb frozen? */
-	wait_queue_head_t	wait_unfrozen;	/* queue for waiting for
-						   sb to be thawed */
-#ifdef CONFIG_DEBUG_LOCK_ALLOC
-	struct lockdep_map	lock_map[SB_FREEZE_LEVELS];
-#endif
+	int				frozen;		/* Is sb frozen? */
+	wait_queue_head_t		wait_unfrozen;	/* for get_super_thawed() */
+	struct percpu_rw_semaphore	rw_sem[SB_FREEZE_LEVELS];
 };
 
 struct super_block {
@@ -1393,9 +1386,9 @@ void __sb_end_write(struct super_block *sb, int level);
 int __sb_start_write(struct super_block *sb, int level, bool wait);
 
 #define __sb_writers_acquired(sb, lev)	\
-	rwsem_acquire_read(&(sb)->s_writers.lock_map[(lev)-1], 0, 1, _THIS_IP_)
+	percpu_rwsem_acquire(&(sb)->s_writers.rw_sem[(lev)-1], 1, _THIS_IP_)
 #define __sb_writers_release(sb, lev)	\
-	rwsem_release(&(sb)->s_writers.lock_map[(lev)-1], 1, _THIS_IP_)
+	percpu_rwsem_release(&(sb)->s_writers.rw_sem[(lev)-1], 1, _THIS_IP_)
 
 /**
  * sb_end_write - drop write access to a superblock