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, 58 insertions, 0 deletions

diff --git a/Documentation/process/programming-language.rst b/Documentation/process/programming-language.rst
new file mode 100644
index 000000000..c18e307cc
--- /dev/null
+++ b/Documentation/process/programming-language.rst
@@ -0,0 +1,58 @@
+.. _programming_language:
+
+Programming Language
+====================
+
+The Linux kernel is written in the C programming language [c-language]_.
+More precisely, it is typically compiled with ``gcc`` [gcc]_
+under ``-std=gnu11`` [gcc-c-dialect-options]_: the GNU dialect of ISO C11.
+``clang`` [clang]_ is also supported; see documentation on
+:ref:`Building Linux with Clang/LLVM <kbuild_llvm>`.
+
+This dialect contains many extensions to the language [gnu-extensions]_,
+and many of them are used within the kernel as a matter of course.
+
+Attributes
+----------
+
+One of the common extensions used throughout the kernel are attributes
+[gcc-attribute-syntax]_. Attributes allow to introduce
+implementation-defined semantics to language entities (like variables,
+functions or types) without having to make significant syntactic changes
+to the language (e.g. adding a new keyword) [n2049]_.
+
+In some cases, attributes are optional (i.e. a compiler not supporting them
+should still produce proper code, even if it is slower or does not perform
+as many compile-time checks/diagnostics).
+
+The kernel defines pseudo-keywords (e.g. ``__pure``) instead of using
+directly the GNU attribute syntax (e.g. ``__attribute__((__pure__))``)
+in order to feature detect which ones can be used and/or to shorten the code.
+
+Please refer to ``include/linux/compiler_attributes.h`` for more information.
+
+Rust
+----
+
+The kernel has support for the Rust programming language
+[rust-language]_ under ``CONFIG_RUST``. It is compiled with ``rustc`` [rustc]_
+under ``--edition=2021`` [rust-editions]_. Editions are a way to introduce
+small changes to the language that are not backwards compatible.
+
+On top of that, some unstable features [rust-unstable-features]_ are used in
+the kernel. Unstable features may change in the future, thus it is an important
+goal to reach a point where only stable features are used.
+
+Please refer to Documentation/rust/index.rst for more information.
+
+.. [c-language] http://www.open-std.org/jtc1/sc22/wg14/www/standards
+.. [gcc] https://gcc.gnu.org
+.. [clang] https://clang.llvm.org
+.. [gcc-c-dialect-options] https://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html
+.. [gnu-extensions] https://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html
+.. [gcc-attribute-syntax] https://gcc.gnu.org/onlinedocs/gcc/Attribute-Syntax.html
+.. [n2049] http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2049.pdf
+.. [rust-language] https://www.rust-lang.org
+.. [rustc] https://doc.rust-lang.org/rustc/
+.. [rust-editions] https://doc.rust-lang.org/edition-guide/editions/
+.. [rust-unstable-features] https://github.com/Rust-for-Linux/linux/issues/2