Re: ipc/sem, ipc/msg, ipc/mqueue.c kcsan questions

[Paul E. McKenney]

On Thu, May 13, 2021 at 12:02:01PM -0700, Paul E. McKenney wrote:
> On Thu, May 13, 2021 at 08:10:51AM +0200, Manfred Spraul wrote:
> > Hi Paul,
> > 
> > On 5/12/21 10:17 PM, Paul E. McKenney wrote:
> > > On Wed, May 12, 2021 at 09:58:18PM +0200, Manfred Spraul wrote:
> > > > [...]
> > > > sma->use_global_lock is evaluated in sem_lock() twice:
> > > > 
> > > > >         /*
> > > > >           * Initial check for use_global_lock. Just an optimization,
> > > > >           * no locking, no memory barrier.
> > > > >           */
> > > > >          if (!sma->use_global_lock) {
> > > > Both sides of the if-clause handle possible data races.
> > > > 
> > > > Is
> > > > 
> > > >      if (!data_race(sma->use_global_lock)) {
> > > > 
> > > > the correct thing to suppress the warning?
> > > Most likely READ_ONCE() rather than data_race(), but please see
> > > the end of this message.
> > 
> > Based on the document, I would say data_race() is sufficient:
> > 
> > I have replaced the code with "if (jiffies %2)", and it runs fine.
> 
> OK, but please note that "jiffies" is marked volatile, which prevents the
> compiler from fusing loads.  You just happen to be OK in this particular
> case, as described below.  Use of the "jiffies_64" non-volatile synonym
> for "jiffies" is better for this sort of checking.  But even so, just
> because a particular version of a particular compiler refrains from
> fusing loads in a particular situation does not mean that all future
> versions of all future compilers will behave so nicely.
> 
> Again, you are OK in this particular situation, as described below.
> 
> > Thus I don't see which evil things a compiler could do, ... .
> 
> Fair enough, and your example is covered by the section "Reads Feeding
> Into Error-Tolerant Heuristics".  The worst that the compiler can do is
> to force an unnecessary acquisition of the global lock.
> 
> This cannot cause incorrect execution, but could results in poor
> scalability.  This could be a problem is load fusing were possible, that
> is, if successes calls to this function were inlined and the compiler
> just reused the value initially loaded.
> 
> The reason that load fusing cannot happen in this case is that the
> load is immediately followed by a lock acquisition, which implies a
> barrier(), which prevents the compiler from fusing loads on opposite
> sides of that barrier().
> 
> > [...]
> > 
> > Does tools/memory-model/Documentation/access-marking.txt, shown below,
> > > help?
> > > 
> > [...]
> > > 	int foo;
> > > 	DEFINE_RWLOCK(foo_rwlock);
> > > 
> > > 	void update_foo(int newval)
> > > 	{
> > > 		write_lock(&foo_rwlock);
> > > 		foo = newval;
> > > 		do_something(newval);
> > > 		write_unlock(&foo_rwlock);
> > > 	}
> > > 
> > > 	int read_foo(void)
> > > 	{
> > > 		int ret;
> > > 
> > > 		read_lock(&foo_rwlock);
> > > 		do_something_else();
> > > 		ret = foo;
> > > 		read_unlock(&foo_rwlock);
> > > 		return ret;
> > > 	}
> > > 
> > > 	int read_foo_diagnostic(void)
> > > 	{
> > > 		return data_race(foo);
> > > 	}
> > 
> > The text didn't help, the example has helped:
> > 
> > It was not clear to me if I have to use data_race() both on the read and the
> > write side, or only on one side.
> > 
> > Based on this example: plain C may be paired with data_race(), there is no
> > need to mark both sides.
> 
> Actually, you just demonstrated that this example is quite misleading.
> That data_race() works only because the read is for diagnostic
> purposes.  I am queuing a commit with your Reported-by that makes
> read_foo_diagnostic() just do a pr_info(), like this:
> 
> 	void read_foo_diagnostic(void)
> 	{
> 		pr_info("Current value of foo: %d\n", data_race(foo));
> 	}
> 
> So thank you for that!

And please see below for an example better illustrating your use case.
Anything messed up or missing?

							Thanx, Paul

------------------------------------------------------------------------

commit b4287410ee93109501defc4695ccc29144e8f3a3
Author: Paul E. McKenney <paulmck@xxxxxxxxxx>
Date:   Thu May 13 14:54:58 2021 -0700

    tools/memory-model: Add example for heuristic lockless reads
    
    This commit adds example code for heuristic lockless reads, based loosely
    on the sem_lock() and sem_unlock() functions.
    
    Reported-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
    Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

diff --git a/tools/memory-model/Documentation/access-marking.txt b/tools/memory-model/Documentation/access-marking.txt
index 58bff2619876..e4a20ebf565d 100644
--- a/tools/memory-model/Documentation/access-marking.txt
+++ b/tools/memory-model/Documentation/access-marking.txt
@@ -319,6 +319,98 @@ 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 sem_lock() and sem_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)
+	{
+		/* IMPORTANT: Heuristic plus spin_lock()! */
+		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_flag);
+		/* Lock held, thus global flag cannot change. */
+		if (!global_flag) {
+			spin_lock(&fp->f_lock);
+			spin_unlock(&global_flag);
+		}
+		do_something(fp);
+		if (global_flag)
+			spin_unlock(&global_flag);
+		else
+			spin_lock(&fp->f_lock);
+	}
+
+	void begin_global(void)
+	{
+		int i;
+
+		spin_lock(&global_flag);
+		WRITE_ONCE(global_flag, true);
+		for (i = 0; i < nfoo; i++) {
+			/* Wait for pre-existing local locks. */
+			spin_lock(&fp->f_lock);
+			spin_unlock(&fp->f_lock);
+		}
+		spin_unlock(&global_flag);
+	}
+
+	void end_global(void)
+	{
+		spin_lock(&global_flag);
+		smp_store_release(&global_flag, false);
+		/* Pre-existing global lock acquisitions will recheck. */
+		spin_unlock(&global_flag);
+	}
+
+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 global_lock, which prevents global_flag from changing.
+If this recheck finds that global_flag is now false, the acquisition
+of ->f_lock prior to the release of global_lock will result in any subsequent
+begin_global() invocation waiting to acquire ->f_lock.
+
+On the other hand, if the value read from global_flag is false, then
+global_flag, then rechecking under ->f_lock combined with synchronization
+with begin_global() guarantees than any erroneous read will cause the
+do_something_locked() function's first do_something() invocation to happen
+before begin_global() returns.  The combination of the smp_load_acquire()
+in do_something_locked() and the smp_store_release() in end_global()
+guarantees that either the do_something_locked() function's first
+do_something() invocation happens after the call to end_global() or that
+do_something_locked() acquires global_lock() and rechecks under the lock.
+
+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 locking each
+and every foo structure.
+
+
 Lockless Reads and Writes
 -------------------------