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

[Paul E. McKenney]

On Fri, May 14, 2021 at 06:01:37PM +0200, Manfred Spraul wrote:
> Hi Paul,
> 
> On 5/14/21 12:01 AM, 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@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.
> 
> I would refer to nf_conntrack_all_lock() instead of sem_lock():
> 
> nf_conntrack_all_lock() is far easier to read, and it contains the same
> heuristics

Sounds good, updated to nf_conntrack_lock(), nf_conntrack_all_lock(),
and nf_conntrack_all_unlock().

> >      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);
> 
> spin_unlock(&global_lock), not &global_flag.
> 
> That was the main results from the discussions a few years ago:
> 
> Split global_lock and global_flag. Do not try to use
> spin_is_locked(&global_lock). Just add a flag. The 4 bytes are well
> invested.

Thank you for catching this typo!  It is now global_lock.

							Thanx, Paul

> > +		}
> > +		do_something(fp);
> > +		if (global_flag)
> > +			spin_unlock(&global_flag);
> &global_lock
> > +		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
> >   -------------------------
> 
>