Rebase locking/kcsan to locking/urgent

Merge the state of the locking kcsan branch before the read/write_once()
and the atomics modifications got merged.

Squash the fallout of the rebase on top of the read/write once and atomic
fallback work into the merge. The history of the original branch is
preserved in tag locking-kcsan-2020-06-02.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

2 files changed, 329 insertions, 0 deletions

diff --git a/Documentation/dev-tools/index.rst b/Documentation/dev-tools/index.rst
index 09dee10d2592..f7809c7b1ba9 100644
--- a/Documentation/dev-tools/index.rst
+++ b/Documentation/dev-tools/index.rst
@@ -21,6 +21,7 @@ whole; patches welcome!
    kasan
    ubsan
    kmemleak
+   kcsan
    gdb-kernel-debugging
    kgdb
    kselftest
diff --git a/Documentation/dev-tools/kcsan.rst b/Documentation/dev-tools/kcsan.rst
new file mode 100644
index 000000000000..f4b5766f12cc
--- /dev/null
+++ b/Documentation/dev-tools/kcsan.rst
@@ -0,0 +1,328 @@
+The Kernel Concurrency Sanitizer (KCSAN)
+========================================
+
+The Kernel Concurrency Sanitizer (KCSAN) is a dynamic race detector, which
+relies on compile-time instrumentation, and uses a watchpoint-based sampling
+approach to detect races. KCSAN's primary purpose is to detect `data races`_.
+
+Usage
+-----
+
+KCSAN is supported in both GCC and Clang. With GCC it requires version 7.3.0 or
+later. With Clang it requires version 7.0.0 or later.
+
+To enable KCSAN configure the kernel with::
+
+    CONFIG_KCSAN = y
+
+KCSAN provides several other configuration options to customize behaviour (see
+the respective help text in ``lib/Kconfig.kcsan`` for more info).
+
+Error reports
+~~~~~~~~~~~~~
+
+A typical data race report looks like this::
+
+    ==================================================================
+    BUG: KCSAN: data-race in generic_permission / kernfs_refresh_inode
+
+    write to 0xffff8fee4c40700c of 4 bytes by task 175 on cpu 4:
+     kernfs_refresh_inode+0x70/0x170
+     kernfs_iop_permission+0x4f/0x90
+     inode_permission+0x190/0x200
+     link_path_walk.part.0+0x503/0x8e0
+     path_lookupat.isra.0+0x69/0x4d0
+     filename_lookup+0x136/0x280
+     user_path_at_empty+0x47/0x60
+     vfs_statx+0x9b/0x130
+     __do_sys_newlstat+0x50/0xb0
+     __x64_sys_newlstat+0x37/0x50
+     do_syscall_64+0x85/0x260
+     entry_SYSCALL_64_after_hwframe+0x44/0xa9
+
+    read to 0xffff8fee4c40700c of 4 bytes by task 166 on cpu 6:
+     generic_permission+0x5b/0x2a0
+     kernfs_iop_permission+0x66/0x90
+     inode_permission+0x190/0x200
+     link_path_walk.part.0+0x503/0x8e0
+     path_lookupat.isra.0+0x69/0x4d0
+     filename_lookup+0x136/0x280
+     user_path_at_empty+0x47/0x60
+     do_faccessat+0x11a/0x390
+     __x64_sys_access+0x3c/0x50
+     do_syscall_64+0x85/0x260
+     entry_SYSCALL_64_after_hwframe+0x44/0xa9
+
+    Reported by Kernel Concurrency Sanitizer on:
+    CPU: 6 PID: 166 Comm: systemd-journal Not tainted 5.3.0-rc7+ #1
+    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
+    ==================================================================
+
+The header of the report provides a short summary of the functions involved in
+the race. It is followed by the access types and stack traces of the 2 threads
+involved in the data race.
+
+The other less common type of data race report looks like this::
+
+    ==================================================================
+    BUG: KCSAN: data-race in e1000_clean_rx_irq+0x551/0xb10
+
+    race at unknown origin, with read to 0xffff933db8a2ae6c of 1 bytes by interrupt on cpu 0:
+     e1000_clean_rx_irq+0x551/0xb10
+     e1000_clean+0x533/0xda0
+     net_rx_action+0x329/0x900
+     __do_softirq+0xdb/0x2db
+     irq_exit+0x9b/0xa0
+     do_IRQ+0x9c/0xf0
+     ret_from_intr+0x0/0x18
+     default_idle+0x3f/0x220
+     arch_cpu_idle+0x21/0x30
+     do_idle+0x1df/0x230
+     cpu_startup_entry+0x14/0x20
+     rest_init+0xc5/0xcb
+     arch_call_rest_init+0x13/0x2b
+     start_kernel+0x6db/0x700
+
+    Reported by Kernel Concurrency Sanitizer on:
+    CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.3.0-rc7+ #2
+    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
+    ==================================================================
+
+This report is generated where it was not possible to determine the other
+racing thread, but a race was inferred due to the data value of the watched
+memory location having changed. These can occur either due to missing
+instrumentation or e.g. DMA accesses. These reports will only be generated if
+``CONFIG_KCSAN_REPORT_RACE_UNKNOWN_ORIGIN=y`` (selected by default).
+
+Selective analysis
+~~~~~~~~~~~~~~~~~~
+
+It may be desirable to disable data race detection for specific accesses,
+functions, compilation units, or entire subsystems.  For static blacklisting,
+the below options are available:
+
+* KCSAN understands the ``data_race(expr)`` annotation, which tells KCSAN that
+  any data races due to accesses in ``expr`` should be ignored and resulting
+  behaviour when encountering a data race is deemed safe.
+
+* Disabling data race detection for entire functions can be accomplished by
+  using the function attribute ``__no_kcsan``::
+
+    __no_kcsan
+    void foo(void) {
+        ...
+
+  To dynamically limit for which functions to generate reports, see the
+  `DebugFS interface`_ blacklist/whitelist feature.
+
+  For ``__always_inline`` functions, replace ``__always_inline`` with
+  ``__no_kcsan_or_inline`` (which implies ``__always_inline``)::
+
+    static __no_kcsan_or_inline void foo(void) {
+        ...
+
+  Note: Older compiler versions (GCC < 9) also do not always honor the
+  ``__no_kcsan`` attribute on regular ``inline`` functions. If false positives
+  with these compilers cannot be tolerated, for small functions where
+  ``__always_inline`` would be appropriate, ``__no_kcsan_or_inline`` should be
+  preferred instead.
+
+* To disable data race detection for a particular compilation unit, add to the
+  ``Makefile``::
+
+    KCSAN_SANITIZE_file.o := n
+
+* To disable data race detection for all compilation units listed in a
+  ``Makefile``, add to the respective ``Makefile``::
+
+    KCSAN_SANITIZE := n
+
+Furthermore, it is possible to tell KCSAN to show or hide entire classes of
+data races, depending on preferences. These can be changed via the following
+Kconfig options:
+
+* ``CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY``: If enabled and a conflicting write
+  is observed via a watchpoint, but the data value of the memory location was
+  observed to remain unchanged, do not report the data race.
+
+* ``CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC``: Assume that plain aligned writes
+  up to word size are atomic by default. Assumes that such writes are not
+  subject to unsafe compiler optimizations resulting in data races. The option
+  causes KCSAN to not report data races due to conflicts where the only plain
+  accesses are aligned writes up to word size.
+
+DebugFS interface
+~~~~~~~~~~~~~~~~~
+
+The file ``/sys/kernel/debug/kcsan`` provides the following interface:
+
+* Reading ``/sys/kernel/debug/kcsan`` returns various runtime statistics.
+
+* Writing ``on`` or ``off`` to ``/sys/kernel/debug/kcsan`` allows turning KCSAN
+  on or off, respectively.
+
+* Writing ``!some_func_name`` to ``/sys/kernel/debug/kcsan`` adds
+  ``some_func_name`` to the report filter list, which (by default) blacklists
+  reporting data races where either one of the top stackframes are a function
+  in the list.
+
+* Writing either ``blacklist`` or ``whitelist`` to ``/sys/kernel/debug/kcsan``
+  changes the report filtering behaviour. For example, the blacklist feature
+  can be used to silence frequently occurring data races; the whitelist feature
+  can help with reproduction and testing of fixes.
+
+Tuning performance
+~~~~~~~~~~~~~~~~~~
+
+Core parameters that affect KCSAN's overall performance and bug detection
+ability are exposed as kernel command-line arguments whose defaults can also be
+changed via the corresponding Kconfig options.
+
+* ``kcsan.skip_watch`` (``CONFIG_KCSAN_SKIP_WATCH``): Number of per-CPU memory
+  operations to skip, before another watchpoint is set up. Setting up
+  watchpoints more frequently will result in the likelihood of races to be
+  observed to increase. This parameter has the most significant impact on
+  overall system performance and race detection ability.
+
+* ``kcsan.udelay_task`` (``CONFIG_KCSAN_UDELAY_TASK``): For tasks, the
+  microsecond delay to stall execution after a watchpoint has been set up.
+  Larger values result in the window in which we may observe a race to
+  increase.
+
+* ``kcsan.udelay_interrupt`` (``CONFIG_KCSAN_UDELAY_INTERRUPT``): For
+  interrupts, the microsecond delay to stall execution after a watchpoint has
+  been set up. Interrupts have tighter latency requirements, and their delay
+  should generally be smaller than the one chosen for tasks.
+
+They may be tweaked at runtime via ``/sys/module/kcsan/parameters/``.
+
+Data Races
+----------
+
+In an execution, two memory accesses form a *data race* if they *conflict*,
+they happen concurrently in different threads, and at least one of them is a
+*plain access*; they *conflict* if both access the same memory location, and at
+least one is a write. For a more thorough discussion and definition, see `"Plain
+Accesses and Data Races" in the LKMM`_.
+
+.. _"Plain Accesses and Data Races" in the LKMM: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/explanation.txt#n1922
+
+Relationship with the Linux-Kernel Memory Consistency Model (LKMM)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The LKMM defines the propagation and ordering rules of various memory
+operations, which gives developers the ability to reason about concurrent code.
+Ultimately this allows to determine the possible executions of concurrent code,
+and if that code is free from data races.
+
+KCSAN is aware of *marked atomic operations* (``READ_ONCE``, ``WRITE_ONCE``,
+``atomic_*``, etc.), but is oblivious of any ordering guarantees and simply
+assumes that memory barriers are placed correctly. In other words, KCSAN
+assumes that as long as a plain access is not observed to race with another
+conflicting access, memory operations are correctly ordered.
+
+This means that KCSAN will not report *potential* data races due to missing
+memory ordering. Developers should therefore carefully consider the required
+memory ordering requirements that remain unchecked. If, however, missing
+memory ordering (that is observable with a particular compiler and
+architecture) leads to an observable data race (e.g. entering a critical
+section erroneously), KCSAN would report the resulting data race.
+
+Race Detection Beyond Data Races
+--------------------------------
+
+For code with complex concurrency design, race-condition bugs may not always
+manifest as data races. Race conditions occur if concurrently executing
+operations result in unexpected system behaviour. On the other hand, data races
+are defined at the C-language level. The following macros can be used to check
+properties of concurrent code where bugs would not manifest as data races.
+
+.. kernel-doc:: include/linux/kcsan-checks.h
+    :functions: ASSERT_EXCLUSIVE_WRITER ASSERT_EXCLUSIVE_WRITER_SCOPED
+                ASSERT_EXCLUSIVE_ACCESS ASSERT_EXCLUSIVE_ACCESS_SCOPED
+                ASSERT_EXCLUSIVE_BITS
+
+Implementation Details
+----------------------
+
+KCSAN relies on observing that two accesses happen concurrently. Crucially, we
+want to (a) increase the chances of observing races (especially for races that
+manifest rarely), and (b) be able to actually observe them. We can accomplish
+(a) by injecting various delays, and (b) by using address watchpoints (or
+breakpoints).
+
+If we deliberately stall a memory access, while we have a watchpoint for its
+address set up, and then observe the watchpoint to fire, two accesses to the
+same address just raced. Using hardware watchpoints, this is the approach taken
+in `DataCollider
+<http://usenix.org/legacy/events/osdi10/tech/full_papers/Erickson.pdf>`_.
+Unlike DataCollider, KCSAN does not use hardware watchpoints, but instead
+relies on compiler instrumentation and "soft watchpoints".
+
+In KCSAN, watchpoints are implemented using an efficient encoding that stores
+access type, size, and address in a long; the benefits of using "soft
+watchpoints" are portability and greater flexibility. KCSAN then relies on the
+compiler instrumenting plain accesses. For each instrumented plain access:
+
+1. Check if a matching watchpoint exists; if yes, and at least one access is a
+   write, then we encountered a racing access.
+
+2. Periodically, if no matching watchpoint exists, set up a watchpoint and
+   stall for a small randomized delay.
+
+3. Also check the data value before the delay, and re-check the data value
+   after delay; if the values mismatch, we infer a race of unknown origin.
+
+To detect data races between plain and marked accesses, KCSAN also annotates
+marked accesses, but only to check if a watchpoint exists; i.e. KCSAN never
+sets up a watchpoint on marked accesses. By never setting up watchpoints for
+marked operations, if all accesses to a variable that is accessed concurrently
+are properly marked, KCSAN will never trigger a watchpoint and therefore never
+report the accesses.
+
+Key Properties
+~~~~~~~~~~~~~~
+
+1. **Memory Overhead:**  The overall memory overhead is only a few MiB
+   depending on configuration. The current implementation uses a small array of
+   longs to encode watchpoint information, which is negligible.
+
+2. **Performance Overhead:** KCSAN's runtime aims to be minimal, using an
+   efficient watchpoint encoding that does not require acquiring any shared
+   locks in the fast-path. For kernel boot on a system with 8 CPUs:
+
+   - 5.0x slow-down with the default KCSAN config;
+   - 2.8x slow-down from runtime fast-path overhead only (set very large
+     ``KCSAN_SKIP_WATCH`` and unset ``KCSAN_SKIP_WATCH_RANDOMIZE``).
+
+3. **Annotation Overheads:** Minimal annotations are required outside the KCSAN
+   runtime. As a result, maintenance overheads are minimal as the kernel
+   evolves.
+
+4. **Detects Racy Writes from Devices:** Due to checking data values upon
+   setting up watchpoints, racy writes from devices can also be detected.
+
+5. **Memory Ordering:** KCSAN is *not* explicitly aware of the LKMM's ordering
+   rules; this may result in missed data races (false negatives).
+
+6. **Analysis Accuracy:** For observed executions, due to using a sampling
+   strategy, the analysis is *unsound* (false negatives possible), but aims to
+   be complete (no false positives).
+
+Alternatives Considered
+-----------------------
+
+An alternative data race detection approach for the kernel can be found in the
+`Kernel Thread Sanitizer (KTSAN) <https://github.com/google/ktsan/wiki>`_.
+KTSAN is a happens-before data race detector, which explicitly establishes the
+happens-before order between memory operations, which can then be used to
+determine data races as defined in `Data Races`_.
+
+To build a correct happens-before relation, KTSAN must be aware of all ordering
+rules of the LKMM and synchronization primitives. Unfortunately, any omission
+leads to large numbers of false positives, which is especially detrimental in
+the context of the kernel which includes numerous custom synchronization
+mechanisms. To track the happens-before relation, KTSAN's implementation
+requires metadata for each memory location (shadow memory), which for each page
+corresponds to 4 pages of shadow memory, and can translate into overhead of
+tens of GiB on a large system.