linux-kernel - [PATCH 2/2] kernfs: remove mutex in kernfs_dop

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Date:   Wed,  2 Dec 2020 22:58:37 +0800
From:   Fox Chen <foxhlchen@...il.com>
To:     gregkh@...uxfoundation.org, tj@...nel.org
Cc:     Fox Chen <foxhlchen@...il.com>, linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] kernfs: remove mutex in kernfs_dop_revalidate

There is a big mutex in kernfs_dop_revalidate which slows down the
concurrent performance of kernfs.

Since kernfs_dop_revalidate only does some checks, the lock is
largely unnecessary. Also, according to kernel filesystem locking
document:
https://www.kernel.org/doc/html/latest/filesystems/locking.html
locking is not in the protocal for d_revalidate operation.

This patch remove this mutex from
kernfs_dop_revalidate, so kernfs_dop_revalidate
can run concurrently.

Signed-off-by: Fox Chen <foxhlchen@...il.com>
---
 fs/kernfs/dir.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 9aec80b9d7c6..c2267c93f546 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -26,7 +26,6 @@ static DEFINE_SPINLOCK(kernfs_idr_lock);	/* root->ino_idr */
 
 static bool kernfs_active(struct kernfs_node *kn)
 {
-	lockdep_assert_held(&kernfs_mutex);
 	return atomic_read(&kn->active) >= 0;
 }
 
@@ -557,10 +556,9 @@ static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)
 
 	/* Always perform fresh lookup for negatives */
 	if (d_really_is_negative(dentry))
-		goto out_bad_unlocked;
+		goto out_bad;
 
 	kn = kernfs_dentry_node(dentry);
-	mutex_lock(&kernfs_mutex);
 
 	/* The kernfs node has been deactivated */
 	if (!kernfs_active(kn))
@@ -579,11 +577,8 @@ static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)
 	    kernfs_info(dentry->d_sb)->ns != kn->ns)
 		goto out_bad;
 
-	mutex_unlock(&kernfs_mutex);
 	return 1;
 out_bad:
-	mutex_unlock(&kernfs_mutex);
-out_bad_unlocked:
 	return 0;
 }
 
@@ -650,6 +645,8 @@ static struct kernfs_node *__kernfs_new_node(struct kernfs_root *root,
 	kn->mode = mode;
 	kn->flags = flags;
 
+	rwlock_init(&kn->iattr_rwlock);
+
 	if (!uid_eq(uid, GLOBAL_ROOT_UID) || !gid_eq(gid, GLOBAL_ROOT_GID)) {
 		struct iattr iattr = {
 			.ia_valid = ATTR_UID | ATTR_GID,
-- 
2.29.2