[PATCH RFC] libfs: Remove parent dentry locking in offset_iterate_dir()

[Chuck Lever <cel@kernel.org>]

From: Chuck Lever <chuck.lever@oracle.com>

Since offset_iterate_dir() does not walk the parent's d_subdir list
nor does it manipulate the parent's d_child, there doesn't seem to
be a reason to hold the parent's d_lock. The offset_ctx's xarray can
be sufficiently protected with just the RCU read lock.

Flame graph data captured during the git regression run shows a
20% reduction in CPU cycles consumed in offset_find_next().

Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202307171640.e299f8d5-oliver.sang@intel.com
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 fs/libfs.c |    9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

This is a possible fix for the will-it-scale regression recently
reported by the kernel test robot. It passes the git regression
test over NFS and doesn't seem to perturb xfstests.

I'm not able to run lkp here yet, so anyone who can run the
will-it-scale test, please report the results. Many thanks.


diff --git a/fs/libfs.c b/fs/libfs.c
index fcc0f1f3c2dc..b69c41fb3c63 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -406,7 +406,7 @@ static struct dentry *offset_find_next(struct xa_state *xas)
 	child = xas_next_entry(xas, U32_MAX);
 	if (!child)
 		goto out;
-	spin_lock_nested(&child->d_lock, DENTRY_D_LOCK_NESTED);
+	spin_lock(&child->d_lock);
 	if (simple_positive(child))
 		found = dget_dlock(child);
 	spin_unlock(&child->d_lock);
@@ -424,17 +424,14 @@ static bool offset_dir_emit(struct dir_context *ctx, struct dentry *dentry)
 			  inode->i_ino, fs_umode_to_dtype(inode->i_mode));
 }
 
-static void offset_iterate_dir(struct dentry *dir, struct dir_context *ctx)
+static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
 {
-	struct inode *inode = d_inode(dir);
 	struct offset_ctx *so_ctx = inode->i_op->get_offset_ctx(inode);
 	XA_STATE(xas, &so_ctx->xa, ctx->pos);
 	struct dentry *dentry;
 
 	while (true) {
-		spin_lock(&dir->d_lock);
 		dentry = offset_find_next(&xas);
-		spin_unlock(&dir->d_lock);
 		if (!dentry)
 			break;
 
@@ -478,7 +475,7 @@ static int offset_readdir(struct file *file, struct dir_context *ctx)
 	if (!dir_emit_dots(file, ctx))
 		return 0;
 
-	offset_iterate_dir(dir, ctx);
+	offset_iterate_dir(d_inode(dir), ctx);
 	return 0;
 }