linux-kernel - [PATCH 2/3] fs/libfs.c: use 'spin_lock_nested' when taking 'd_lock' for dentry in simple

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Date:   Sat, 30 Nov 2019 10:02:24 +0800
From:   yu kuai <yukuai3@...wei.com>
To:     <gregkh@...uxfoundation.org>, <rafael@...nel.org>,
        <viro@...iv.linux.org.uk>, <rostedt@...dmis.org>,
        <oleg@...hat.com>, <mchehab+samsung@...nel.org>, <corbet@....net>,
        <tytso@....edu>, <jmorris@...ei.org>
CC:     <linux-kernel@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
        <yukuai3@...wei.com>, <zhengbin13@...wei.com>,
        <yi.zhang@...wei.com>, <chenxiang66@...ilicon.com>,
        <xiexiuqi@...wei.com>
Subject: [PATCH 2/3] fs/libfs.c: use 'spin_lock_nested' when taking 'd_lock' for dentry in simple_empty

simple_emtpty currently use 'spin_lock_nested' for 'child' to avoid
confusion for lockdep. However, it's not used for 'dentry'.

In that case, there will be a problem if the caller called 'simple_empty'
with a parent's 'd_lock' held:

spin_lock(&dentry->d_parent->d_lock)
    call simple_empty(dentry)
        spin_lock(&dentry->d_lock) --> lockdep will report this
            spin_lock_nested(child->d_lock, spin_lock_nested)
            spin_unlock(child_lock)
        spin_unlock($dentry->d_lock)
    return from simple_empty
spin_unlock(&dentry->d_patrent->d_lock)

So, use 'DENTRY_D_LOCK_NESTED' for 'dentry' and 'DENTRY_D_LOCK_NESTED_TWICE'
for child.

Signed-off-by: yu kuai <yukuai3@...wei.com>
---
 fs/libfs.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/libfs.c b/fs/libfs.c
index 1463b038ffc4..54a37444f7f9 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -336,9 +336,9 @@ int simple_empty(struct dentry *dentry)
 	struct dentry *child;
 	int ret = 0;
 
-	spin_lock(&dentry->d_lock);
+	spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
 	list_for_each_entry(child, &dentry->d_subdirs, d_child) {
-		spin_lock_nested(&child->d_lock, DENTRY_D_LOCK_NESTED);
+		spin_lock_nested(&child->d_lock, DENTRY_D_LOCK_NESTED_TWICE);
 		if (simple_positive(child)) {
 			spin_unlock(&child->d_lock);
 			goto out;
-- 
2.17.2