[PATCH RFC] ext4: skip concurrent inode updates in lazytime optimization

[Konstantin Khlebnikov <khlebnikov@yandex-team.ru>]

Function ext4_update_other_inodes_time() implements optimization which
opportunistically updates times for inodes within same inode table block.

For now	concurrent inode lookup by number does not scale well because
inode hash table is protected with single spinlock. It could become very
hot at concurrent writes to fast nvme when inode cache has enough inodes.

Probably someday inode hash will become searchable under RCU.
(see linked patchset by David Howells)

Let's skip concurrent updates instead of wasting cpu time at spinlock.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Link: https://lore.kernel.org/lkml/155620449631.4720.8762546550728087460.stgit@warthog.procyon.org.uk/
---
 fs/ext4/inode.c |    7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 629a25d999f0..dc3e1b38e3ed 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -4849,11 +4849,16 @@ static int other_inode_match(struct inode * inode, unsigned long ino,
 static void ext4_update_other_inodes_time(struct super_block *sb,
 					  unsigned long orig_ino, char *buf)
 {
+	static DEFINE_SPINLOCK(lock);
 	struct other_inode oi;
 	unsigned long ino;
 	int i, inodes_per_block = EXT4_SB(sb)->s_inodes_per_block;
 	int inode_size = EXT4_INODE_SIZE(sb);
 
+	/* Don't bother inode_hash_lock with concurrent updates. */
+	if (!spin_trylock(&lock))
+		return;
+
 	oi.orig_ino = orig_ino;
 	/*
 	 * Calculate the first inode in the inode table block.  Inode
@@ -4867,6 +4872,8 @@ static void ext4_update_other_inodes_time(struct super_block *sb,
 		oi.raw_inode = (struct ext4_inode *) buf;
 		(void) find_inode_nowait(sb, ino, other_inode_match, &oi);
 	}
+
+	spin_unlock(&lock);
 }
 
 /*