ftrace: Synchronize the initialization of ftrace_opsgrafted

There's some internal state that ftrace_ops needs to have set, but since
it can be declared outside of the ftrace.c code, it calls
ftrace_ops_init() on the ops in every global function. The issue is that
if two tasks call it on the same ops at the same time it is possible to
have the initialization of one corrupt the initialization of the other
call.

Create a ops_mutex to use to synchronize every initialization of the
ftrace_ops. The mutex is taken within checking the ftrace_ops flag that
states it was initializied but the flag is checked again after the mutex
has been taken. Checking first outside the mutex allows it to shortcut
having to take the mutex. But then the check needs to be done again after
the mute is taken in case of races.

Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260902095501.6b59af20@gandalf.local.home
Fixes: f04f24fb7e48d ("ftrace, kprobes: Fix a deadlock on ftrace_regex_lock")
Reported-by: sashiko-bot@kernel.org
Close: https://lore.kernel.org/all/20260829025528.49A831F000E9@smtp.kernel.org/
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 76 insertions, 0 deletions

diff --git a/Documentation/filesystems/virtiofs.rst b/Documentation/filesystems/virtiofs.rst
new file mode 100644
index 000000000..fd4d2484e
--- /dev/null
+++ b/Documentation/filesystems/virtiofs.rst
@@ -0,0 +1,76 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _virtiofs_index:
+
+===================================================
+virtiofs: virtio-fs host<->guest shared file system
+===================================================
+
+- Copyright (C) 2019 Red Hat, Inc.
+
+Introduction
+============
+The virtiofs file system for Linux implements a driver for the paravirtualized
+VIRTIO "virtio-fs" device for guest<->host file system sharing.  It allows a
+guest to mount a directory that has been exported on the host.
+
+Guests often require access to files residing on the host or remote systems.
+Use cases include making files available to new guests during installation,
+booting from a root file system located on the host, persistent storage for
+stateless or ephemeral guests, and sharing a directory between guests.
+
+Although it is possible to use existing network file systems for some of these
+tasks, they require configuration steps that are hard to automate and they
+expose the storage network to the guest.  The virtio-fs device was designed to
+solve these problems by providing file system access without networking.
+
+Furthermore the virtio-fs device takes advantage of the co-location of the
+guest and host to increase performance and provide semantics that are not
+possible with network file systems.
+
+Usage
+=====
+Mount file system with tag ``myfs`` on ``/mnt``:
+
+.. code-block:: sh
+
+  guest# mount -t virtiofs myfs /mnt
+
+Please see https://virtio-fs.gitlab.io/ for details on how to configure QEMU
+and the virtiofsd daemon.
+
+Mount options
+-------------
+
+virtiofs supports general VFS mount options, for example, remount,
+ro, rw, context, etc. It also supports FUSE mount options.
+
+atime behavior
+^^^^^^^^^^^^^^
+
+The atime-related mount options, for example, noatime, strictatime,
+are ignored. The atime behavior for virtiofs is the same as the
+underlying filesystem of the directory that has been exported
+on the host.
+
+Internals
+=========
+Since the virtio-fs device uses the FUSE protocol for file system requests, the
+virtiofs file system for Linux is integrated closely with the FUSE file system
+client.  The guest acts as the FUSE client while the host acts as the FUSE
+server.  The /dev/fuse interface between the kernel and userspace is replaced
+with the virtio-fs device interface.
+
+FUSE requests are placed into a virtqueue and processed by the host.  The
+response portion of the buffer is filled in by the host and the guest handles
+the request completion.
+
+Mapping /dev/fuse to virtqueues requires solving differences in semantics
+between /dev/fuse and virtqueues.  Each time the /dev/fuse device is read, the
+FUSE client may choose which request to transfer, making it possible to
+prioritize certain requests over others.  Virtqueues have queue semantics and
+it is not possible to change the order of requests that have been enqueued.
+This is especially important if the virtqueue becomes full since it is then
+impossible to add high priority requests.  In order to address this difference,
+the virtio-fs device uses a "hiprio" virtqueue specifically for requests that
+have priority over normal requests.