eventfs: Remove expectation that ei->is_freed means ei->dentry == NULL

The logic to free the eventfs_inode (ei) use to set is_freed and clear the
"dentry" field under the eventfs_mutex. But that changed when a race was
found where the ei->dentry needed to be cleared when the last dput() was
called on it. But there was still logic that checked if ei->dentry was not
NULL and is_freed is set, and would warn if it was.

But since that situation was changed and the ei->dentry isn't cleared
until the last dput() is called on it while the ei->is_freed is set, do
not test for that condition anymore, and change the comments to reflect
that.

Link: https://lkml.kernel.org/r/20231120235154.265826243@goodmis.org

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Fixes: 020010fb ("eventfs: Delete eventfs_inode when the last dentry is freed")
Reported-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

fs/tracefs/event_inode.c

@@ -27,16 +27,16 @@
 /*
  * eventfs_mutex protects the eventfs_inode (ei) dentry. Any access
  * to the ei->dentry must be done under this mutex and after checking
- * if ei->is_freed is not set. The ei->dentry is released under the
- * mutex at the same time ei->is_freed is set. If ei->is_freed is set
- * then the ei->dentry is invalid.
+ * if ei->is_freed is not set. When ei->is_freed is set, the dentry
+ * is on its way to being freed after the last dput() is made on it.
  */
 static DEFINE_MUTEX(eventfs_mutex);
 
 /*
  * The eventfs_inode (ei) itself is protected by SRCU. It is released from
  * its parent's list and will have is_freed set (under eventfs_mutex).
- * After the SRCU grace period is over, the ei may be freed.
+ * After the SRCU grace period is over and the last dput() is called
+ * the ei is freed.
  */
 DEFINE_STATIC_SRCU(eventfs_srcu);
 
@@ -365,12 +365,14 @@ create_file_dentry(struct eventfs_inode *ei, int idx,
 		 * created the dentry for this e_dentry. In which case
 		 * use that one.
 		 *
-		 * Note, with the mutex held, the e_dentry cannot have content
-		 * and the ei->is_freed be true at the same time.
+		 * If ei->is_freed is set, the e_dentry is currently on its
+		 * way to being freed, don't return it. If e_dentry is NULL
+		 * it means it was already freed.
 		 */
-		dentry = *e_dentry;
-		if (WARN_ON_ONCE(dentry && ei->is_freed))
+		if (ei->is_freed)
 			dentry = NULL;
+		else
+			dentry = *e_dentry;
 		/* The lookup does not need to up the dentry refcount */
 		if (dentry && !lookup)
 			dget(dentry);
@@ -473,8 +475,8 @@ create_dir_dentry(struct eventfs_inode *pei, struct eventfs_inode *ei,
 		 * created the dentry for this e_dentry. In which case
 		 * use that one.
 		 *
-		 * Note, with the mutex held, the e_dentry cannot have content
-		 * and the ei->is_freed be true at the same time.
+		 * If ei->is_freed is set, the e_dentry is currently on its
+		 * way to being freed.
 		 */
 		dentry = ei->dentry;
 		if (dentry && !lookup)