ext4: for committing inode, make ext4_fc_track_inode wait

If the inode that's being requested to track using ext4_fc_track_inode
is being committed, then wait until the inode finishes the
commit. Also, add calls to ext4_fc_track_inode at the right places.
With this patch, now calling ext4_reserve_inode_write() results in
inode being tracked for next fast commit. This ensures that by the
time ext4_reserve_inode_write() returns, it is ready to be modified
and won't be committed until the corresponding handle is open.

A subtle lock ordering requirement with i_data_sem (which is
documented in the code) requires that ext4_fc_track_inode() be called
before grabbing i_data_sem. So, this patch also adds explicit
ext4_fc_track_inode() calls in places where i_data_sem grabbed.

Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Link: https://patch.msgid.link/20250508175908.1004880-3-harshadshirwadkar@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

1 files changed, 35 insertions, 0 deletions

diff --git a/fs/ext4/fast_commit.c b/fs/ext4/fast_commit.c
index 63859ec6d91d..c4d3c71d5e6c 100644
--- a/fs/ext4/fast_commit.c
+++ b/fs/ext4/fast_commit.c
@@ -12,6 +12,7 @@
 #include "ext4_extents.h"
 #include "mballoc.h"
 
+#include <linux/lockdep.h>
 /*
  * Ext4 Fast Commits
  * -----------------
@@ -570,6 +571,8 @@ static int __track_inode(handle_t *handle, struct inode *inode, void *arg,
 
 void ext4_fc_track_inode(handle_t *handle, struct inode *inode)
 {
+	struct ext4_inode_info *ei = EXT4_I(inode);
+	wait_queue_head_t *wq;
 	int ret;
 
 	if (S_ISDIR(inode->i_mode))
@@ -587,6 +590,38 @@ void ext4_fc_track_inode(handle_t *handle, struct inode *inode)
 	if (ext4_test_mount_flag(inode->i_sb, EXT4_MF_FC_INELIGIBLE))
 		return;
 
+	if (!list_empty(&ei->i_fc_list))
+		return;
+
+	/*
+	 * If we come here, we may sleep while waiting for the inode to
+	 * commit. We shouldn't be holding i_data_sem when we go to sleep since
+	 * the commit path needs to grab the lock while committing the inode.
+	 */
+	lockdep_assert_not_held(&ei->i_data_sem);
+
+	while (ext4_test_inode_state(inode, EXT4_STATE_FC_COMMITTING)) {
+#if (BITS_PER_LONG < 64)
+		DEFINE_WAIT_BIT(wait, &ei->i_state_flags,
+				EXT4_STATE_FC_COMMITTING);
+		wq = bit_waitqueue(&ei->i_state_flags,
+				   EXT4_STATE_FC_COMMITTING);
+#else
+		DEFINE_WAIT_BIT(wait, &ei->i_flags,
+				EXT4_STATE_FC_COMMITTING);
+		wq = bit_waitqueue(&ei->i_flags,
+				   EXT4_STATE_FC_COMMITTING);
+#endif
+		prepare_to_wait(wq, &wait.wq_entry, TASK_UNINTERRUPTIBLE);
+		if (ext4_test_inode_state(inode, EXT4_STATE_FC_COMMITTING))
+			schedule();
+		finish_wait(wq, &wait.wq_entry);
+	}
+
+	/*
+	 * From this point on, this inode will not be committed either
+	 * by fast or full commit as long as the handle is open.
+	 */
 	ret = ext4_fc_track_template(handle, inode, __track_inode, NULL, 1);
 	trace_ext4_fc_track_inode(handle, inode, ret);
 }