[PATCH AUTOSEL 5.15 08/10] debugfs: add API to allow debugfs operations cancellation

Sasha Levin
2023-12-04T20:48:38Z

From: Johannes Berg <johannes.berg@intel.com>

[ Upstream commit 8c88a474357ead632b07c70bf7f119ace8c3b39e ]

In some cases there might be longer-running hardware accesses
in debugfs files, or attempts to acquire locks, and we want
to still be able to quickly remove the files.

Introduce a cancellations API to use inside the debugfs handler
functions to be able to cancel such operations on a per-file
basis.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/debugfs/file.c | 82 +++++++++++++++++++++++++++++++++++++++++
fs/debugfs/inode.c | 32 +++++++++++++++-
fs/debugfs/internal.h | 5 +++
include/linux/debugfs.h | 19 ++++++++++
4 files changed, 137 insertions(+), 1 deletion(-)

diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
index a1aea4d1637c7..a1144f1de8c36 100644
--- a/fs/debugfs/file.c
+++ b/fs/debugfs/file.c
@@ -114,6 +114,8 @@ int debugfs_file_get(struct dentry *dentry)
lockdep_init_map(&fsd->lockdep_map, fsd->lock_name ?: "debugfs",
&fsd->key, 0);
#endif
+ INIT_LIST_HEAD(&fsd->cancellations);
+ mutex_init(&fsd->cancellations_mtx);
}

/*
@@ -156,6 +158,86 @@ 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 014ff204548d9..48cb009139945 100644
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -225,6 +225,8 @@ static void debugfs_release_dentry(struct dentry *dentry)
lockdep_unregister_key(&fsd->key);
kfree(fsd->lock_name);
#endif
+ WARN_ON(!list_empty(&fsd->cancellations));
+ mutex_destroy(&fsd->cancellations_mtx);
}

kfree(fsd);
@@ -724,8 +726,36 @@ static void __debugfs_file_removed(struct dentry *dentry)
lock_map_acquire(&fsd->lockdep_map);
lock_map_release(&fsd->lockdep_map);

- 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 c7d61cfc97d26..0c4c68cf161f8 100644
--- a/fs/debugfs/internal.h
+++ b/fs/debugfs/internal.h
@@ -8,6 +8,7 @@
#ifndef _DEBUGFS_INTERNAL_H_
#define _DEBUGFS_INTERNAL_H_
#include <linux/lockdep.h>
+#include <linux/list.h>

struct file_operations;

@@ -29,6 +30,10 @@ struct debugfs_fsdata {
struct lock_class_key key;
char *lock_name;
#endif
+
+ /* protect cancellations */
+ struct mutex cancellations_mtx;
+ struct list_head cancellations;
};
};
};
diff --git a/include/linux/debugfs.h b/include/linux/debugfs.h
index ea2d919fd9c79..c9c65b132c0fd 100644
--- a/include/linux/debugfs.h
+++ b/include/linux/debugfs.h
@@ -171,6 +171,25 @@ ssize_t debugfs_write_file_bool(struct file *file, const char __user *user_buf,
ssize_t debugfs_read_file_str(struct file *file, char __user *user_buf,
size_t count, loff_t *ppos);

+/**
+ * struct debugfs_cancellation - cancellation data
+ * @list: internal, for keeping track
+ * @cancel: callback to call
+ * @cancel_data: extra data for the callback to call
+ */
+struct debugfs_cancellation {
+ struct list_head list;
+ void (*cancel)(struct dentry *, void *);
+ void *cancel_data;
+};
+
+void __acquires(cancellation)
+debugfs_enter_cancellation(struct file *file,
+ struct debugfs_cancellation *cancellation);
+void __releases(cancellation)
+debugfs_leave_cancellation(struct file *file,
+ struct debugfs_cancellation *cancellation);
+
#else

#include <linux/err.h>
--
2.42.0