linux-ext4 - [PATCH v9 9/9] ext4: hold s_fc_lock while during fast commit

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Message-ID: <20250508175908.1004880-10-harshadshirwadkar@gmail.com>
Date: Thu,  8 May 2025 17:59:08 +0000
From: Harshad Shirwadkar <harshadshirwadkar@...il.com>
To: linux-ext4@...r.kernel.org
Cc: tytso@....edu,
	jack@...e.cz,
	harshads@...gle.com,
	Harshad Shirwadkar <harshadshirwadkar@...il.com>
Subject: [PATCH v9 9/9] ext4: hold s_fc_lock while during fast commit

Leaving s_fc_lock in between during commit in ext4_fc_perform_commit()
function leaves room for subtle concurrency bugs where ext4_fc_del() may
delete an inode from the fast commit list, leaving list in an inconsistent
state.

Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@...il.com>
Reviewed-by: Jan Kara <jack@...e.cz>
---
 fs/ext4/fast_commit.c | 44 +++++++++++++------------------------------
 1 file changed, 13 insertions(+), 31 deletions(-)

diff --git a/fs/ext4/fast_commit.c b/fs/ext4/fast_commit.c
index eb888e522..7ac672e35 100644
--- a/fs/ext4/fast_commit.c
+++ b/fs/ext4/fast_commit.c
@@ -424,6 +424,7 @@ static int __track_dentry_update(handle_t *handle, struct inode *inode,
 	node->fcd_ino = inode->i_ino;
 	take_dentry_name_snapshot(&node->fcd_name, dentry);
 	INIT_LIST_HEAD(&node->fcd_dilist);
+	INIT_LIST_HEAD(&node->fcd_list);
 	mutex_lock(&sbi->s_fc_lock);
 	if (sbi->s_journal->j_flags & JBD2_FULL_COMMIT_ONGOING ||
 		sbi->s_journal->j_flags & JBD2_FAST_COMMIT_ONGOING)
@@ -985,8 +986,6 @@ static int ext4_fc_flush_data(journal_t *journal)
 
 /* Commit all the directory entry updates */
 static int ext4_fc_commit_dentry_updates(journal_t *journal, u32 *crc)
-__acquires(&sbi->s_fc_lock)
-__releases(&sbi->s_fc_lock)
 {
 	struct super_block *sb = journal->j_private;
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
@@ -1000,26 +999,22 @@ __releases(&sbi->s_fc_lock)
 	list_for_each_entry_safe(fc_dentry, fc_dentry_n,
 				 &sbi->s_fc_dentry_q[FC_Q_MAIN], fcd_list) {
 		if (fc_dentry->fcd_op != EXT4_FC_TAG_CREAT) {
-			mutex_unlock(&sbi->s_fc_lock);
-			if (!ext4_fc_add_dentry_tlv(sb, crc, fc_dentry)) {
-				ret = -ENOSPC;
-				goto lock_and_exit;
-			}
-			mutex_lock(&sbi->s_fc_lock);
+			if (!ext4_fc_add_dentry_tlv(sb, crc, fc_dentry))
+				return -ENOSPC;
 			continue;
 		}
 		/*
 		 * With fcd_dilist we need not loop in sbi->s_fc_q to get the
-		 * corresponding inode pointer
+		 * corresponding inode. Also, the corresponding inode could have been
+		 * deleted, in which case, we don't need to do anything.
 		 */
-		WARN_ON(list_empty(&fc_dentry->fcd_dilist));
+		if (list_empty(&fc_dentry->fcd_dilist))
+			continue;
 		ei = list_first_entry(&fc_dentry->fcd_dilist,
 				struct ext4_inode_info, i_fc_dilist);
 		inode = &ei->vfs_inode;
 		WARN_ON(inode->i_ino != fc_dentry->fcd_ino);
 
-		mutex_unlock(&sbi->s_fc_lock);
-
 		/*
 		 * We first write the inode and then the create dirent. This
 		 * allows the recovery code to create an unnamed inode first
@@ -1029,23 +1024,14 @@ __releases(&sbi->s_fc_lock)
 		 */
 		ret = ext4_fc_write_inode(inode, crc);
 		if (ret)
-			goto lock_and_exit;
-
+			return ret;
 		ret = ext4_fc_write_inode_data(inode, crc);
 		if (ret)
-			goto lock_and_exit;
-
-		if (!ext4_fc_add_dentry_tlv(sb, crc, fc_dentry)) {
-			ret = -ENOSPC;
-			goto lock_and_exit;
-		}
-
-		mutex_lock(&sbi->s_fc_lock);
+			return ret;
+		if (!ext4_fc_add_dentry_tlv(sb, crc, fc_dentry))
+			return -ENOSPC;
 	}
 	return 0;
-lock_and_exit:
-	mutex_lock(&sbi->s_fc_lock);
-	return ret;
 }
 
 static int ext4_fc_perform_commit(journal_t *journal)
@@ -1148,10 +1134,8 @@ static int ext4_fc_perform_commit(journal_t *journal)
 	/* Step 6.2: Now write all the dentry updates. */
 	mutex_lock(&sbi->s_fc_lock);
 	ret = ext4_fc_commit_dentry_updates(journal, &crc);
-	if (ret) {
-		mutex_unlock(&sbi->s_fc_lock);
+	if (ret)
 		goto out;
-	}
 
 	/* Step 6.3: Now write all the changed inodes to disk. */
 	list_for_each_entry(iter, &sbi->s_fc_q[FC_Q_MAIN], i_fc_list) {
@@ -1159,7 +1143,6 @@ static int ext4_fc_perform_commit(journal_t *journal)
 		if (!ext4_test_inode_state(inode, EXT4_STATE_FC_COMMITTING))
 			continue;
 
-		mutex_unlock(&sbi->s_fc_lock);
 		ret = ext4_fc_write_inode_data(inode, &crc);
 		if (ret)
 			goto out;
@@ -1171,6 +1154,7 @@ static int ext4_fc_perform_commit(journal_t *journal)
 	ret = ext4_fc_write_tail(sb, crc);
 
 out:
+	mutex_unlock(&sbi->s_fc_lock);
 	blk_finish_plug(&plug);
 	return ret;
 }
@@ -1353,11 +1337,9 @@ static void ext4_fc_cleanup(journal_t *journal, int full, tid_t tid)
 					     fcd_list);
 		list_del_init(&fc_dentry->fcd_list);
 		list_del_init(&fc_dentry->fcd_dilist);
-		mutex_unlock(&sbi->s_fc_lock);
 
 		release_dentry_name_snapshot(&fc_dentry->fcd_name);
 		kmem_cache_free(ext4_fc_dentry_cachep, fc_dentry);
-		mutex_lock(&sbi->s_fc_lock);
 	}
 
 	list_splice_init(&sbi->s_fc_dentry_q[FC_Q_STAGING],
-- 
2.49.0.1045.g170613ef41-goog