thunderbolt: Fix NULL dereference in tb_remove_work()grafted

There is a slight race between tb_remove_work() and tb_domain_remove()
which leads to dereferencing a NULL tb->root_switch pointer inside
tb_free_unplugged_xdomains():

        Thread A                            Thread B

  tb_remove_work()
                                    tb_domain_remove()
                                      mutex_lock(&tb->lock)
                                      tb_stop()
                                        /* doesn't cancel a running callback */
                                        cancel_delayed_work(&tcm->remove_work)
                                        ...
                                        tb_switch_remove(tb->root_switch)
                                        tb->root_switch = NULL
                                      mutex_unlock(&tb->lock)
    mutex_lock(&tb->lock)
    ...
    /* without checking ->root_switch */
    tb_free_unplugged_xdomains(tb->root_switch)
    mutex_unlock(&tb->lock)

Commit a8937f35cf39 ("thunderbolt: Remove XDomain from the bus without
holding tb->lock") doesn't seem right to move tb_free_unplugged_xdomains()
out of the &tb->lock section and the check for tb->root_switch, in
particular.  It states:

  For this reason separate removing the XDomain from the topology data
  structures (where we need the lock) from unregistering the device from
  the bus (where remove callbacks of the drivers are being called).

tb_free_unplugged_xdomains() belongs to the former group of functions
requiring the lock.  And it also calls tb_xdomain_remove() which should
only be called with &tb->lock held.

Found by Linux Verification Center (linuxtesting.org) with Svace static
analysis tool.

Fixes: a8937f35cf39 ("thunderbolt: Remove XDomain from the bus without holding tb->lock")
Cc: stable@vger.kernel.org
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>

1 files changed, 95 insertions, 0 deletions

diff --git a/Documentation/driver-api/pwrseq.rst b/Documentation/driver-api/pwrseq.rst
new file mode 100644
index 000000000..e1056d141
--- /dev/null
+++ b/Documentation/driver-api/pwrseq.rst
@@ -0,0 +1,95 @@
+.. SPDX-License-Identifier: GPL-2.0-only
+.. Copyright 2024 Linaro Ltd.
+
+====================
+Power Sequencing API
+====================
+
+:Author: Bartosz Golaszewski
+
+Introduction
+============
+
+This framework is designed to abstract complex power-up sequences that are
+shared between multiple logical devices in the Linux kernel.
+
+The intention is to allow consumers to obtain a power sequencing handle
+exposed by the power sequence provider and delegate the actual requesting and
+control of the underlying resources as well as to allow the provider to
+mitigate any potential conflicts between multiple users behind the scenes.
+
+Glossary
+--------
+
+The power sequencing API uses a number of terms specific to the subsystem:
+
+Unit
+
+    A unit is a discrete chunk of a power sequence. For instance one unit may
+    enable a set of regulators, another may enable a specific GPIO. Units can
+    define dependencies in the form of other units that must be enabled before
+    it itself can be.
+
+Target
+
+    A target is a set of units (composed of the "final" unit and its
+    dependencies) that a consumer selects by its name when requesting a handle
+    to the power sequencer. Via the dependency system, multiple targets may
+    share the same parts of a power sequence but ignore parts that are
+    irrelevant.
+
+Descriptor
+
+    A handle passed by the pwrseq core to every consumer that serves as the
+    entry point to the provider layer. It ensures coherence between different
+    users and keeps reference counting consistent.
+
+Consumer interface
+==================
+
+The consumer API is aimed to be as simple as possible. The driver interested in
+getting a descriptor from the power sequencer should call pwrseq_get() and
+specify the name of the target it wants to reach in the sequence after calling
+pwrseq_enable(). The descriptor can be released by calling pwrseq_put() and
+the consumer can request the powering down of its target with
+pwrseq_disable(). Note that there is no guarantee that pwrseq_disable()
+will have any effect as there may be multiple users of the underlying resources
+who may keep them active.
+
+Provider interface
+==================
+
+The provider API is admittedly not nearly as straightforward as the one for
+consumers but it makes up for it in flexibility.
+
+Each provider can logically split the power-up sequence into discrete chunks
+(units) and define their dependencies. They can then expose named targets that
+consumers may use as the final point in the sequence that they wish to reach.
+
+To that end the providers fill out a set of configuration structures and
+register with the pwrseq subsystem by calling pwrseq_device_register().
+
+Dynamic consumer matching
+-------------------------
+
+The main difference between pwrseq and other Linux kernel providers is the
+mechanism for dynamic matching of consumers and providers. Every power sequence
+provider driver must implement the `match()` callback and pass it to the pwrseq
+core when registering with the subsystems.
+
+When a client requests a sequencer handle, the core will call this callback for
+every registered provider and let it flexibly figure out whether the proposed
+client device is indeed its consumer. For example: if the provider binds to the
+device-tree node representing a power management unit of a chipset and the
+consumer driver controls one of its modules, the provider driver may parse the
+relevant regulator supply properties in device tree and see if they lead from
+the PMU to the consumer.
+
+API reference
+=============
+
+.. kernel-doc:: include/linux/pwrseq/provider.h
+   :internal:
+
+.. kernel-doc:: drivers/power/sequencing/core.c
+   :export: