Date:   Fri, 14 May 2021 11:47:11 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Hillf Danton <hdanton@...a.com>
Cc:     Manfred Spraul <manfred@...orfullife.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Davidlohr Bueso <dbueso@...e.de>, 1vier1@....de
Subject: Re: ipc/sem, ipc/msg, ipc/mqueue.c kcsan questions

On Fri, May 14, 2021 at 04:29:18PM +0800, Hillf Danton wrote:
> On Thu, 13 May 2021 15:01:27 Paul E. McKenney wrote:
> >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@...nel.org>
> >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@...orfullife.com>
> >    Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> >
> >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)
> 
> The global flag may change without global lock held - we will likely have the
> wrong lock released if we can see the change.

Right you are!  I am adding a local variable to address this, thank you!

							Thanx, Paul

> >+			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
> > -------------------------
> > 
> >

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