ext4: improve smp scalability for inode generation

->s_next_generation is protected by s_next_gen_lock but its usage
pattern is very primitive.  We don't actually need sequentially
increasing new generation numbers, so let's use prandom_u32() instead.

Reported-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/ioctl.c

@@ -14,6 +14,7 @@
 #include <linux/mount.h>
 #include <linux/file.h>
 #include <linux/quotaops.h>
+#include <linux/random.h>
 #include <linux/uuid.h>
 #include <linux/uaccess.h>
 #include <linux/delay.h>
@@ -98,7 +99,6 @@ static long swap_inode_boot_loader(struct super_block *sb,
 	int err;
 	struct inode *inode_bl;
 	struct ext4_inode_info *ei_bl;
-	struct ext4_sb_info *sbi = EXT4_SB(sb);
 
 	if (inode->i_nlink != 1 || !S_ISREG(inode->i_mode))
 		return -EINVAL;
@@ -157,10 +157,8 @@ static long swap_inode_boot_loader(struct super_block *sb,
 
 	inode->i_ctime = inode_bl->i_ctime = current_time(inode);
 
-	spin_lock(&sbi->s_next_gen_lock);
-	inode->i_generation = sbi->s_next_generation++;
-	inode_bl->i_generation = sbi->s_next_generation++;
-	spin_unlock(&sbi->s_next_gen_lock);
+	inode->i_generation = prandom_u32();
+	inode_bl->i_generation = prandom_u32();
 
 	ext4_discard_preallocations(inode);