Merge branch 'lkmm' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu into locking/debug

Pull LKMM changes from Paul E. McKenney:

  "These changes focus on documentation, providing additional
   examples and use cases."

Signed-off-by: Ingo Molnar <mingo@kernel.org>

Author: ingomolnar

tools/memory-model/Documentation/access-marking.txt

@@ -37,7 +37,9 @@ compiler's use of code-motion and common-subexpression optimizations.
 Therefore, if a given access is involved in an intentional data race,
 using READ_ONCE() for loads and WRITE_ONCE() for stores is usually
 preferable to data_race(), which in turn is usually preferable to plain
-C-language accesses.
+C-language accesses.  It is permissible to combine #2 and #3, for example,
+data_race(READ_ONCE(a)), which will both restrict compiler optimizations
+and disable KCSAN diagnostics.
 
 KCSAN will complain about many types of data races involving plain
 C-language accesses, but marking all accesses involved in a given data
@@ -86,6 +88,10 @@ that fail to exclude the updates.  In this case, it is important to use
 data_race() for the diagnostic reads because otherwise KCSAN would give
 false-positive warnings about these diagnostic reads.
 
+If it is necessary to both restrict compiler optimizations and disable
+KCSAN diagnostics, use both data_race() and READ_ONCE(), for example,
+data_race(READ_ONCE(a)).
+
 In theory, plain C-language loads can also be used for this use case.
 However, in practice this will have the disadvantage of causing KCSAN
 to generate false positives because KCSAN will have no way of knowing
@@ -126,6 +132,11 @@ consistent errors, which in turn are quite capable of breaking heuristics.
 Therefore use of data_race() should be limited to cases where some other
 code (such as a barrier() call) will force the occasional reload.
 
+Note that this use case requires that the heuristic be able to handle
+any possible error.  In contrast, if the heuristics might be fatally
+confused by one or more of the possible erroneous values, use READ_ONCE()
+instead of data_race().
+
 In theory, plain C-language loads can also be used for this use case.
 However, in practice this will have the disadvantage of causing KCSAN
 to generate false positives because KCSAN will have no way of knowing
@@ -259,9 +270,9 @@ diagnostic purposes.  The code might look as follows:
 		return ret;
 	}
 
-	int read_foo_diagnostic(void)
+	void read_foo_diagnostic(void)
 	{
-		return data_race(foo);
+		pr_info("Current value of foo: %d\n", data_race(foo));
 	}
 
 The reader-writer lock prevents the compiler from introducing concurrency
@@ -274,19 +285,34 @@ tells KCSAN that data races are expected, and should be silently
 ignored.  This data_race() also tells the human reading the code that
 read_foo_diagnostic() might sometimes return a bogus value.
 
-However, please note that your kernel must be built with
-CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC=n in order for KCSAN to
-detect a buggy lockless write.  If you need KCSAN to detect such a
-write even if that write did not change the value of foo, you also
-need CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY=n.  If you need KCSAN to
-detect such a write happening in an interrupt handler running on the
-same CPU doing the legitimate lock-protected write, you also need
-CONFIG_KCSAN_INTERRUPT_WATCHER=y.  With some or all of these Kconfig
-options set properly, KCSAN can be quite helpful, although it is not
-necessarily a full replacement for hardware watchpoints.  On the other
-hand, neither are hardware watchpoints a full replacement for KCSAN
-because it is not always easy to tell hardware watchpoint to conditionally
-trap on accesses.
+If it is necessary to suppress compiler optimization and also detect
+buggy lockless writes, read_foo_diagnostic() can be updated as follows:
+
+	void read_foo_diagnostic(void)
+	{
+		pr_info("Current value of foo: %d\n", data_race(READ_ONCE(foo)));
+	}
+
+Alternatively, given that KCSAN is to ignore all accesses in this function,
+this function can be marked __no_kcsan and the data_race() can be dropped:
+
+	void __no_kcsan read_foo_diagnostic(void)
+	{
+		pr_info("Current value of foo: %d\n", READ_ONCE(foo));
+	}
+
+However, in order for KCSAN to detect buggy lockless writes, your kernel
+must be built with CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC=n.  If you
+need KCSAN to detect such a write even if that write did not change
+the value of foo, you also need CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY=n.
+If you need KCSAN to detect such a write happening in an interrupt handler
+running on the same CPU doing the legitimate lock-protected write, you
+also need CONFIG_KCSAN_INTERRUPT_WATCHER=y.  With some or all of these
+Kconfig options set properly, KCSAN can be quite helpful, although
+it is not necessarily a full replacement for hardware watchpoints.
+On the other hand, neither are hardware watchpoints a full replacement
+for KCSAN because it is not always easy to tell hardware watchpoint to
+conditionally trap on accesses.
 
 
 Lock-Protected Writes With Lockless Reads
@@ -319,6 +345,99 @@ of the ASSERT_EXCLUSIVE_WRITER() is to allow KCSAN to check for a buggy
 concurrent lockless write.
 
 
+Lock-Protected Writes With Heuristic Lockless Reads
+---------------------------------------------------
+
+For another example, suppose that the code can normally make use of
+a per-data-structure lock, but there are times when a global lock
+is required.  These times are indicated via a global flag.  The code
+might look as follows, and is based loosely on nf_conntrack_lock(),
+nf_conntrack_all_lock(), and nf_conntrack_all_unlock():
+
+	bool global_flag;
+	DEFINE_SPINLOCK(global_lock);
+	struct foo {
+		spinlock_t f_lock;
+		int f_data;
+	};
+
+	/* All foo structures are in the following array. */
+	int nfoo;
+	struct foo *foo_array;
+
+	void do_something_locked(struct foo *fp)
+	{
+		/* This works even if data_race() returns nonsense. */
+		if (!data_race(global_flag)) {
+			spin_lock(&fp->f_lock);
+			if (!smp_load_acquire(&global_flag)) {
+				do_something(fp);
+				spin_unlock(&fp->f_lock);
+				return;
+			}
+			spin_unlock(&fp->f_lock);
+		}
+		spin_lock(&global_lock);
+		/* global_lock held, thus global flag cannot be set. */
+		spin_lock(&fp->f_lock);
+		spin_unlock(&global_lock);
+		/*
+		 * global_flag might be set here, but begin_global()
+		 * will wait for ->f_lock to be released.
+		 */
+		do_something(fp);
+		spin_unlock(&fp->f_lock);
+	}
+
+	void begin_global(void)
+	{
+		int i;
+
+		spin_lock(&global_lock);
+		WRITE_ONCE(global_flag, true);
+		for (i = 0; i < nfoo; i++) {
+			/*
+			 * Wait for pre-existing local locks.  One at
+			 * a time to avoid lockdep limitations.
+			 */
+			spin_lock(&fp->f_lock);
+			spin_unlock(&fp->f_lock);
+		}
+	}
+
+	void end_global(void)
+	{
+		smp_store_release(&global_flag, false);
+		spin_unlock(&global_lock);
+	}
+
+All code paths leading from the do_something_locked() function's first
+read from global_flag acquire a lock, so endless load fusing cannot
+happen.
+
+If the value read from global_flag is true, then global_flag is
+rechecked while holding ->f_lock, which, if global_flag is now false,
+prevents begin_global() from completing.  It is therefore safe to invoke
+do_something().
+
+Otherwise, if either value read from global_flag is true, then after
+global_lock is acquired global_flag must be false.  The acquisition of
+->f_lock will prevent any call to begin_global() from returning, which
+means that it is safe to release global_lock and invoke do_something().
+
+For this to work, only those foo structures in foo_array[] may be passed
+to do_something_locked().  The reason for this is that the synchronization
+with begin_global() relies on momentarily holding the lock of each and
+every foo structure.
+
+The smp_load_acquire() and smp_store_release() are required because
+changes to a foo structure between calls to begin_global() and
+end_global() are carried out without holding that structure's ->f_lock.
+The smp_load_acquire() and smp_store_release() ensure that the next
+invocation of do_something() from do_something_locked() will see those
+changes.
+
+
 Lockless Reads and Writes
 -------------------------