[PATCH 3/4] libunload: A library to help remove open files

[Mandeep Sandhu]

The problem of how to remove open files due to module unloading or
device hotunplugging keeps coming up.  We have multiple implementations
of roughly the same logic in proc, sysctl, sysfs, tun and now I am
working on yet another one for uio. It is time to start working on a
generic implementation.

This library does not aim to allow wrapping any arbitray set of file
operations and making it safe to unload any module. This library aims
to work in conjunction with the code implementiong an object to make it
safe to remove the object while file handles to it are still open.
libunload implements the necessary locking and logic to make it
striaght forward to implement file_operations for objects that are
removed at runtime.

It is hard to arrange for the ->close method of vm_operations_struct to
be called when an object is being removed, and this code doesn't even
attempt to help with that. Instead it is assumed that calling ->close
is not needed. Without close support mmap at hotunplug time is simply a
matter of calling umap_mapping_range() to invaildate the mappings, and
to arrange for vm_fault to return VM_FAULT_SIGBUS when the
unload_trylock fails.

Wait queues and fasync queues can safely be woken up after
unload_barrier making the semantics clean. The fasync entries can be
freed as a list of all of the file descriptors is kept. poll entries
can not be freed so the poll wait queue heads must be kept around. If
someone else's poll method is being wrapped, the wrapped poll wait
queue head could be freed, but it requires that there is a wrapping
wait queue head that is kept around. If there is no other way wrapping
a poll wait queue head seems practical but in general it isn't
particularly useful.

libunload is best understood from the perspective of code that calls
unload_barrier(). Past the unload barrier it is guaranteed that there
is no code in the critical sections protectecd by the unload lock, and
the unload release lock. Past the unload barrier it is safe to call the
release methods for remaining file descriptors, to ensure some logical
state does not persist.

Tested-by: Mandeep Sandhu <mandeep.sandhu@xxxxxxxxxxx>
Signed-off-by: Mandeep Sandhu <mandeep.sandhu@xxxxxxxxxxx>
---
 fs/Makefile            |   2 +-
 fs/libunload.c         | 169 +++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/unload.h |  35 ++++++++++
 3 files changed, 205 insertions(+), 1 deletion(-)
 create mode 100644 fs/libunload.c
 create mode 100644 include/linux/unload.h

diff --git a/fs/Makefile b/fs/Makefile
index bedff48..165bcfa 100644
--- a/fs/Makefile
+++ b/fs/Makefile
@@ -11,7 +11,7 @@ obj-y :=	open.o read_write.o file_table.o super.o \
 		attr.o bad_inode.o file.o filesystems.o namespace.o \
 		seq_file.o xattr.o libfs.o fs-writeback.o \
 		pnode.o splice.o sync.o utimes.o \
-		stack.o fs_struct.o statfs.o fs_pin.o nsfs.o
+		stack.o fs_struct.o statfs.o fs_pin.o nsfs.o libunload.o
 
 ifeq ($(CONFIG_BLOCK),y)
 obj-y +=	buffer.o block_dev.o direct-io.o mpage.o
diff --git a/fs/libunload.c b/fs/libunload.c
new file mode 100644
index 0000000..0a365bb
--- /dev/null
+++ b/fs/libunload.c
@@ -0,0 +1,169 @@
+#include <linux/fs.h>
+#include <linux/mm_types.h>
+#include <linux/mm.h>
+#include <linux/spinlock.h>
+#include <linux/module.h>
+#include <linux/unload.h>
+
+struct unload_barrier {
+	struct completion   completion;
+	int         releasers;
+};
+
+void unload_init(struct unload *unload)
+{
+	INIT_HLIST_HEAD(&unload->ufiles);
+	spin_lock_init(&unload->lock);
+	unload->active = 1;
+	unload->barrier = NULL;
+}
+EXPORT_SYMBOL_GPL(unload_init);
+
+void unload_file_init(struct unload_file *ufile,
+		      struct file *file,
+		      struct unload *unload)
+{
+	ufile->file = file;
+	ufile->unload = unload;
+	INIT_HLIST_NODE(&ufile->list);
+}
+EXPORT_SYMBOL_GPL(unload_file_init);
+
+bool unload_trylock(struct unload *unload)
+{
+	bool locked = false;
+
+	spin_lock(&unload->lock);
+	if (likely(!unload->barrier)) {
+		unload->active++;
+		locked = true;
+	}
+	spin_unlock(&unload->lock);
+	return locked;
+}
+EXPORT_SYMBOL_GPL(unload_trylock);
+
+static void __unload_unlock(struct unload *unload)
+{
+	unload->active--;
+	if ((unload->active == 0) && (unload->barrier->releasers == 0))
+		complete(&unload->barrier->completion);
+}
+
+void unload_unlock(struct unload *unload)
+{
+	spin_lock(&unload->lock);
+	__unload_unlock(unload);
+	spin_unlock(&unload->lock);
+}
+EXPORT_SYMBOL_GPL(unload_unlock);
+
+static void __unload_file_attach(struct unload_file *ufile,
+				 struct unload *unload)
+{
+	ufile->unload = unload;
+	hlist_add_head(&ufile->list, &unload->ufiles);
+}
+
+void unload_file_attach(struct unload_file *ufile, struct unload *unload)
+{
+	spin_lock(&unload->lock);
+	__unload_file_attach(ufile, unload);
+	spin_unlock(&unload->lock);
+}
+EXPORT_SYMBOL_GPL(unload_file_attach);
+
+static void __unload_file_detach(struct unload_file *ufile)
+{
+	hlist_del_init(&ufile->list);
+}
+
+void unload_file_detach(struct unload_file *ufile)
+{
+	struct unload *unload = ufile->unload;
+
+	spin_lock(&unload->lock);
+	__unload_file_detach(ufile);
+	spin_unlock(&unload->lock);
+}
+EXPORT_SYMBOL_GPL(unload_file_detach);
+
+struct unload_file *find_unload_file(struct unload *unload, struct file *file)
+{
+	struct unload_file *ufile;
+
+	spin_lock(&unload->lock);
+	hlist_for_each_entry(ufile, &unload->ufiles, list) {
+		if (ufile->file == file)
+			goto done;
+	}
+	ufile = NULL;
+done:
+	spin_unlock(&unload->lock);
+	return ufile;
+}
+EXPORT_SYMBOL_GPL(find_unload_file);
+
+bool unload_release_trylock(struct unload_file *ufile)
+{
+	struct unload *unload = ufile->unload;
+	bool locked = false;
+
+	spin_lock(&unload->lock);
+	if (!hlist_unhashed(&ufile->list))
+		locked = true;
+	spin_unlock(&unload->lock);
+	return locked;
+}
+EXPORT_SYMBOL_GPL(unload_release_trylock);
+
+void unload_release_unlock(struct unload_file *ufile)
+{
+	struct unload *unload = ufile->unload;
+	struct unload_barrier *barrier;
+
+	spin_lock(&unload->lock);
+	__unload_file_detach(ufile);
+	barrier = unload->barrier;
+	if (barrier) {
+		barrier->releasers -= 1;
+		if ((barrier->releasers == 0) && (unload->active == 0))
+			complete(&barrier->completion);
+	}
+	spin_unlock(&unload->lock);
+}
+EXPORT_SYMBOL_GPL(unload_release_unlock);
+
+
+void unload_barrier(struct unload *unload)
+{
+	struct unload_barrier barrier;
+	struct unload_file *ufile;
+
+	/* Guarantee that when this function returns I am not
+	 * executing any code protected by the unload_lock or
+	 * unload_releas_lock, and that I will never again execute
+	 * code protected by those locks.
+	 *
+	 * Also guarantee the file count for every file remaining on
+	 * the unload ufiles list has been incremented.  The increment
+	 * of the file count guarantees __fput will not be called.
+	 */
+	init_completion(&barrier.completion);
+	barrier.releasers = 0;
+
+	spin_lock(&unload->lock);
+	unload->barrier = &barrier;
+
+	hlist_for_each_entry(ufile, &unload->ufiles, list)
+		if (!atomic_long_inc_not_zero(&ufile->file->f_count))
+			barrier.releasers++;
+	unload->active--;
+	if (unload->active || barrier.releasers) {
+		spin_unlock(&unload->lock);
+		wait_for_completion(&barrier.completion);
+		spin_lock(&unload->lock);
+	}
+	spin_unlock(&unload->lock);
+}
+EXPORT_SYMBOL_GPL(unload_barrier);
diff --git a/include/linux/unload.h b/include/linux/unload.h
new file mode 100644
index 0000000..83d378f
--- /dev/null
+++ b/include/linux/unload.h
@@ -0,0 +1,35 @@
+#ifndef _LINUX_UNLOAD_H
+#define _LINUX_UNLOAD_H
+
+#include <linux/list.h>
+
+struct file;
+struct vm_operations_struct;
+struct unload_barrier;
+
+struct unload {
+	struct hlist_head   ufiles;
+	struct unload_barrier   *barrier;
+	spinlock_t      lock;
+	int         active;
+};
+
+struct unload_file {
+	struct unload       *unload;
+	struct hlist_node   list;
+	struct file         *file;
+};
+
+void unload_init(struct unload *unload);
+void unload_file_init(struct unload_file *ufile,
+		      struct file *file,
+		      struct unload *unload);
+bool unload_trylock(struct unload *unload);
+void unload_unlock(struct unload *unload);
+bool unload_release_trylock(struct unload_file *ufile);
+void unload_release_unlock(struct unload_file *ufile);
+void unload_file_attach(struct unload_file *ufile, struct unload *unload);
+void unload_file_detach(struct unload_file *ufile);
+struct unload_file *find_unload_file(struct unload *unload, struct file *file);
+void unload_barrier(struct unload *unload);
+#endif /* _LINUX_UNLOAD_H */
-- 
1.9.1

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