kcsan: Document modeling of weak memory

Document how KCSAN models a subset of weak memory and the subset of
missing memory barriers it can detect as a result.
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/dev-tools/kcsan.rst

@@ -204,17 +204,17 @@ 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.
+``atomic_*``, etc.), and a subset of ordering guarantees implied by memory
+barriers. With ``CONFIG_KCSAN_WEAK_MEMORY=y``, KCSAN models load or store
+buffering, and can detect missing ``smp_mb()``, ``smp_wmb()``, ``smp_rmb()``,
+``smp_store_release()``, and all ``atomic_*`` operations with equivalent
+implied barriers.
+
+Note, KCSAN will not report all data races due to missing memory ordering,
+specifically where a memory barrier would be required to prohibit subsequent
+memory operation from reordering before the barrier. Developers should
+therefore carefully consider the required memory ordering requirements that
+remain unchecked.
 
 Race Detection Beyond Data Races
 --------------------------------
@@ -268,6 +268,56 @@ 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.
 
+Modeling Weak Memory
+~~~~~~~~~~~~~~~~~~~~
+
+KCSAN's approach to detecting data races due to missing memory barriers is
+based on modeling access reordering (with ``CONFIG_KCSAN_WEAK_MEMORY=y``).
+Each plain memory access for which a watchpoint is set up, is also selected for
+simulated reordering within the scope of its function (at most 1 in-flight
+access).
+
+Once an access has been selected for reordering, it is checked along every
+other access until the end of the function scope. If an appropriate memory
+barrier is encountered, the access will no longer be considered for simulated
+reordering.
+
+When the result of a memory operation should be ordered by a barrier, KCSAN can
+then detect data races where the conflict only occurs as a result of a missing
+barrier. Consider the example::
+
+    int x, flag;
+    void T1(void)
+    {
+        x = 1;                  // data race!
+        WRITE_ONCE(flag, 1);    // correct: smp_store_release(&flag, 1)
+    }
+    void T2(void)
+    {
+        while (!READ_ONCE(flag));   // correct: smp_load_acquire(&flag)
+        ... = x;                    // data race!
+    }
+
+When weak memory modeling is enabled, KCSAN can consider ``x`` in ``T1`` for
+simulated reordering. After the write of ``flag``, ``x`` is again checked for
+concurrent accesses: because ``T2`` is able to proceed after the write of
+``flag``, a data race is detected. With the correct barriers in place, ``x``
+would not be considered for reordering after the proper release of ``flag``,
+and no data race would be detected.
+
+Deliberate trade-offs in complexity but also practical limitations mean only a
+subset of data races due to missing memory barriers can be detected. With
+currently available compiler support, the implementation is limited to modeling
+the effects of "buffering" (delaying accesses), since the runtime cannot
+"prefetch" accesses. Also recall that watchpoints are only set up for plain
+accesses, and the only access type for which KCSAN simulates reordering. This
+means reordering of marked accesses is not modeled.
+
+A consequence of the above is that acquire operations do not require barrier
+instrumentation (no prefetching). Furthermore, marked accesses introducing
+address or control dependencies do not require special handling (the marked
+access cannot be reordered, later dependent accesses cannot be prefetched).
+
 Key Properties
 ~~~~~~~~~~~~~~
 
@@ -290,8 +340,8 @@ Key Properties
 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).
+5. **Memory Ordering:** KCSAN is aware of only a subset of LKMM 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