3 files changed, 126 insertions, 2 deletions

diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
index e40229c47f..6d7c1a4958 100644
--- a/fs/debugfs/file.c
+++ b/fs/debugfs/file.c
@@ -104,7 +104,11 @@ int debugfs_file_get(struct dentry *dentry)
 					~DEBUGFS_FSDATA_IS_REAL_FOPS_BIT);
 		refcount_set(&fsd->active_users, 1);
 		init_completion(&fsd->active_users_drained);
+		INIT_LIST_HEAD(&fsd->cancellations);
+		mutex_init(&fsd->cancellations_mtx);
+
 		if (cmpxchg(&dentry->d_fsdata, d_fsd, fsd) != d_fsd) {
+			mutex_destroy(&fsd->cancellations_mtx);
 			kfree(fsd);
 			fsd = READ_ONCE(dentry->d_fsdata);
 		}
@@ -146,6 +150,86 @@ void debugfs_file_put(struct dentry *dentry)
 }
 EXPORT_SYMBOL_GPL(debugfs_file_put);
 
+/**
+ * debugfs_enter_cancellation - enter a debugfs cancellation
+ * @file: the file being accessed
+ * @cancellation: the cancellation object, the cancel callback
+ *	inside of it must be initialized
+ *
+ * When a debugfs file is removed it needs to wait for all active
+ * operations to complete. However, the operation itself may need
+ * to wait for hardware or completion of some asynchronous process
+ * or similar. As such, it may need to be cancelled to avoid long
+ * waits or even deadlocks.
+ *
+ * This function can be used inside a debugfs handler that may
+ * need to be cancelled. As soon as this function is called, the
+ * cancellation's 'cancel' callback may be called, at which point
+ * the caller should proceed to call debugfs_leave_cancellation()
+ * and leave the debugfs handler function as soon as possible.
+ * Note that the 'cancel' callback is only ever called in the
+ * context of some kind of debugfs_remove().
+ *
+ * This function must be paired with debugfs_leave_cancellation().
+ */
+void debugfs_enter_cancellation(struct file *file,
+				struct debugfs_cancellation *cancellation)
+{
+	struct debugfs_fsdata *fsd;
+	struct dentry *dentry = F_DENTRY(file);
+
+	INIT_LIST_HEAD(&cancellation->list);
+
+	if (WARN_ON(!d_is_reg(dentry)))
+		return;
+
+	if (WARN_ON(!cancellation->cancel))
+		return;
+
+	fsd = READ_ONCE(dentry->d_fsdata);
+	if (WARN_ON(!fsd ||
+		    ((unsigned long)fsd & DEBUGFS_FSDATA_IS_REAL_FOPS_BIT)))
+		return;
+
+	mutex_lock(&fsd->cancellations_mtx);
+	list_add(&cancellation->list, &fsd->cancellations);
+	mutex_unlock(&fsd->cancellations_mtx);
+
+	/* if we're already removing wake it up to cancel */
+	if (d_unlinked(dentry))
+		complete(&fsd->active_users_drained);
+}
+EXPORT_SYMBOL_GPL(debugfs_enter_cancellation);
+
+/**
+ * debugfs_leave_cancellation - leave cancellation section
+ * @file: the file being accessed
+ * @cancellation: the cancellation previously registered with
+ *	debugfs_enter_cancellation()
+ *
+ * See the documentation of debugfs_enter_cancellation().
+ */
+void debugfs_leave_cancellation(struct file *file,
+				struct debugfs_cancellation *cancellation)
+{
+	struct debugfs_fsdata *fsd;
+	struct dentry *dentry = F_DENTRY(file);
+
+	if (WARN_ON(!d_is_reg(dentry)))
+		return;
+
+	fsd = READ_ONCE(dentry->d_fsdata);
+	if (WARN_ON(!fsd ||
+		    ((unsigned long)fsd & DEBUGFS_FSDATA_IS_REAL_FOPS_BIT)))
+		return;
+
+	mutex_lock(&fsd->cancellations_mtx);
+	if (!list_empty(&cancellation->list))
+		list_del(&cancellation->list);
+	mutex_unlock(&fsd->cancellations_mtx);
+}
+EXPORT_SYMBOL_GPL(debugfs_leave_cancellation);
+
 /*
  * Only permit access to world-readable files when the kernel is locked down.
  * We also need to exclude any file that has ways to write or alter it as root
diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
index dcde4199a6..034a617cb1 100644
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -72,7 +72,7 @@ static struct inode *debugfs_get_inode(struct super_block *sb)
 	struct inode *inode = new_inode(sb);
 	if (inode) {
 		inode->i_ino = get_next_ino();
-		inode->i_atime = inode->i_mtime = inode_set_ctime_current(inode);
+		simple_inode_init_ts(inode);
 	}
 	return inode;
 }
@@ -241,6 +241,12 @@ static void debugfs_release_dentry(struct dentry *dentry)
 	if ((unsigned long)fsd & DEBUGFS_FSDATA_IS_REAL_FOPS_BIT)
 		return;
 
+	/* check it wasn't a dir (no fsdata) or automount (no real_fops) */
+	if (fsd && fsd->real_fops) {
+		WARN_ON(!list_empty(&fsd->cancellations));
+		mutex_destroy(&fsd->cancellations_mtx);
+	}
+
 	kfree(fsd);
 }
 
@@ -744,8 +750,37 @@ static void __debugfs_file_removed(struct dentry *dentry)
 	fsd = READ_ONCE(dentry->d_fsdata);
 	if ((unsigned long)fsd & DEBUGFS_FSDATA_IS_REAL_FOPS_BIT)
 		return;
-	if (!refcount_dec_and_test(&fsd->active_users))
+
+	/* if we hit zero, just wait for all to finish */
+	if (!refcount_dec_and_test(&fsd->active_users)) {
 		wait_for_completion(&fsd->active_users_drained);
+		return;
+	}
+
+	/* if we didn't hit zero, try to cancel any we can */
+	while (refcount_read(&fsd->active_users)) {
+		struct debugfs_cancellation *c;
+
+		/*
+		 * Lock the cancellations. Note that the cancellations
+		 * structs are meant to be on the stack, so we need to
+		 * ensure we either use them here or don't touch them,
+		 * and debugfs_leave_cancellation() will wait for this
+		 * to be finished processing before exiting one. It may
+		 * of course win and remove the cancellation, but then
+		 * chances are we never even got into this bit, we only
+		 * do if the refcount isn't zero already.
+		 */
+		mutex_lock(&fsd->cancellations_mtx);
+		while ((c = list_first_entry_or_null(&fsd->cancellations,
+						     typeof(*c), list))) {
+			list_del_init(&c->list);
+			c->cancel(dentry, c->cancel_data);
+		}
+		mutex_unlock(&fsd->cancellations_mtx);
+
+		wait_for_completion(&fsd->active_users_drained);
+	}
 }
 
 static void remove_one(struct dentry *victim)
diff --git a/fs/debugfs/internal.h b/fs/debugfs/internal.h
index f7c489b5a3..dae80c2a46 100644
--- a/fs/debugfs/internal.h
+++ b/fs/debugfs/internal.h
@@ -7,6 +7,7 @@
 
 #ifndef _DEBUGFS_INTERNAL_H_
 #define _DEBUGFS_INTERNAL_H_
+#include <linux/list.h>
 
 struct file_operations;
 
@@ -23,6 +24,10 @@ struct debugfs_fsdata {
 		struct {
 			refcount_t active_users;
 			struct completion active_users_drained;
+
+			/* protect cancellations */
+			struct mutex cancellations_mtx;
+			struct list_head cancellations;
 		};
 	};
 };