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/gpu/amdgpu/ring-buffer.rst b/Documentation/gpu/amdgpu/ring-buffer.rst
new file mode 100644
index 000000000..cc642c213
--- /dev/null
+++ b/Documentation/gpu/amdgpu/ring-buffer.rst
@@ -0,0 +1,95 @@
+=============
+ Ring Buffer
+=============
+
+To handle communication between user space and kernel space, AMD GPUs use a
+ring buffer design to feed the engines (GFX, Compute, SDMA, UVD, VCE, VCN, VPE,
+etc.). See the figure below that illustrates how this communication works:
+
+.. kernel-figure:: ring_buffers.svg
+
+Ring buffers in the amdgpu work as a producer-consumer model, where userspace
+acts as the producer, constantly filling the ring buffer with GPU commands to
+be executed. Meanwhile, the GPU retrieves the information from the ring, parses
+it, and distributes the specific set of instructions between the different
+amdgpu blocks.
+
+Notice from the diagram that the ring has a Read Pointer (rptr), which
+indicates where the engine is currently reading packets from the ring, and a
+Write Pointer (wptr), which indicates how many packets software has added to
+the ring. When the rptr and wptr are equal, the ring is idle. When software
+adds packets to the ring, it updates the wptr, this causes the engine to start
+fetching and processing packets. As the engine processes packets, the rptr gets
+updates until the rptr catches up to the wptr and they are equal again.
+
+Usually, ring buffers in the driver have a limited size (search for occurrences
+of `amdgpu_ring_init()`). One of the reasons for the small ring buffer size is
+that CP (Command Processor) is capable of following addresses inserted into the
+ring; this is illustrated in the image by the reference to the IB (Indirect
+Buffer). The IB gives userspace the possibility to have an area in memory that
+CP can read and feed the hardware with extra instructions.
+
+All ASICs pre-GFX11 use what is called a kernel queue, which means
+the ring is allocated in kernel space and has some restrictions, such as not
+being able to be :ref:`preempted directly by the scheduler<amdgpu-mes>`. GFX11
+and newer support kernel queues, but also provide a new mechanism named
+:ref:`user queues<amdgpu-userq>`, where the queue is moved to the user space
+and can be mapped and unmapped via the scheduler. In practice, both queues
+insert user-space-generated GPU commands from different jobs into the requested
+component ring.
+
+Enforce Isolation
+=================
+
+.. note:: After reading this section, you might want to check the
+   :ref:`Process Isolation<amdgpu-process-isolation>` page for more details.
+
+Before examining the Enforce Isolation mechanism in the ring buffer context, it
+is helpful to briefly discuss how instructions from the ring buffer are
+processed in the graphics pipeline. Let’s expand on this topic by checking the
+diagram below that illustrates the graphics pipeline:
+
+.. kernel-figure:: gfx_pipeline_seq.svg
+
+In terms of executing instructions, the GFX pipeline follows the sequence:
+Shader Export (SX), Geometry Engine (GE), Shader Process or Input (SPI), Scan
+Converter (SC), Primitive Assembler (PA), and cache manipulation (which may
+vary across ASICs). Another common way to describe the pipeline is to use Pixel
+Shader (PS), raster, and Vertex Shader (VS) to symbolize the two shader stages.
+Now, with this pipeline in mind, let's assume that Job B causes a hang issue,
+but Job C's instruction might already be executing, leading developers to
+incorrectly identify Job C as the problematic one. This problem can be
+mitigated on multiple levels; the diagram below illustrates how to minimize
+part of this problem:
+
+.. kernel-figure:: no_enforce_isolation.svg
+
+Note from the diagram that there is no guarantee of order or a clear separation
+between instructions, which is not a problem most of the time, and is also good
+for performance. Furthermore, notice some circles between jobs in the diagram
+that represent a **fence wait** used to avoid overlapping work in the ring. At
+the end of the fence, a cache flush occurs, ensuring that when the next job
+starts, it begins in a clean state and, if issues arise, the developer can
+pinpoint the problematic process more precisely.
+
+To increase the level of isolation between jobs, there is the "Enforce
+Isolation" method described in the picture below:
+
+.. kernel-figure:: enforce_isolation.svg
+
+As shown in the diagram, enforcing isolation introduces ordering between
+submissions, since the access to GFX/Compute is serialized, think about it as
+single process at a time mode for gfx/compute. Notice that this approach has a
+significant performance impact, as it allows only one job to submit commands at
+a time. However, this option can help pinpoint the job that caused the problem.
+Although enforcing isolation improves the situation, it does not fully resolve
+the issue of precisely pinpointing bad jobs, since isolation might mask the
+problem. In summary, identifying which job caused the issue may not be precise,
+but enforcing isolation might help with the debugging.
+
+Ring Operations
+===============
+
+.. kernel-doc:: drivers/gpu/drm/amd/amdgpu/amdgpu_ring.c
+   :internal:
+