Re: [PATCH] Convert struct pid count to refcount_t

["Paul E. McKenney" <paulmck@linux.ibm.com>]

On Fri, Mar 29, 2019 at 10:36:39PM -0400, Joel Fernandes wrote:
> On Thu, Mar 28, 2019 at 10:37:07AM -0700, Paul E. McKenney wrote:
> > On Thu, Mar 28, 2019 at 05:26:42PM +0100, Oleg Nesterov wrote:
> > > On 03/28, Jann Horn wrote:
> > > >
> > > > Since we're just talking about RCU stuff now, adding Paul McKenney to
> > > > the thread.
> > > 
> > > Since you added Paul let me add more confusion to this thread ;)
> > 
> > Woo-hoo!!!  More confusion!  Bring it on!!!  ;-)
> 
> Nice to take part in the confusion fun too!!! ;-)
> 
> > > There were some concerns about the lack of barriers in put_pid(), but I can't
> > > find that old discussion and I forgot the result of that discussion...
> > > 
> > > Paul, could you confirm that this code
> > > 
> > > 	CPU_0		CPU_1
> > > 
> > > 	X = 1;		if (READ_ONCE(Y))
> > > 	mb();			X = 2;
> > > 	Y = 1;		BUG_ON(X != 2);
> > > 
> > > 
> > > is correct? I think it is, control dependency pairs with mb(), right?
> > 
> > The BUG_ON() is supposed to happen at the end of time, correct?
> > As written, there is (in the strict sense) a data race between the load
> > of X in the BUG_ON() and CPU_0's store to X.  In a less strict sense,
> > you could of course argue that this data race is harmless, especially
> > if X is a single byte.  But the more I talk to compiler writers, the
> > less comfortable I become with data races in general.  :-/
> > 
> > So I would also feel better if the "Y = 1" was WRITE_ONCE().
> > 
> > On the other hand, this is a great opportunity to try out Alan Stern's
> > prototype plain-accesses patch to the Linux Kernel Memory Model (LKMM)!
> > 
> > https://lkml.kernel.org/r/Pine.LNX.4.44L0.1903191459270.1593-200000@iolanthe.rowland.org
> > 
> > Also adding Alan on CC.
> > 
> > Here is what I believe is the litmus test that your are interested in:
> > 
> > ------------------------------------------------------------------------
> > C OlegNesterov-put_pid
> > 
> > {}
> > 
> > P0(int *x, int *y)
> > {
> > 	*x = 1;
> > 	smp_mb();
> > 	*y = 1;
> > }
> > 
> > P1(int *x, int *y)
> > {
> > 	int r1;
> > 
> > 	r1 = READ_ONCE(*y);
> > 	if (r1)
> > 		*x = 2;
> > }
> > 
> > exists (1:r1=1 /\ ~x=2)
> > ------------------------------------------------------------------------
> > 
> > Running this through herd with Alan's patch detects the data race
> > and says that the undesired outcome is allowed:
> > 
> > 	$ herd7  -conf linux-kernel.cfg /tmp/OlegNesterov-put_pid.litmus 
> > 	Test OlegNesterov-put_pid Allowed
> > 	States 3
> > 	1:r1=0; x=1;
> > 	1:r1=1; x=1;
> > 	1:r1=1; x=2;
> > 	Ok
> > 	Witnesses
> > 	Positive: 1 Negative: 2
> > 	Flag data-race
> > 	Condition exists (1:r1=1 /\ not (x=2))
> > 	Observation OlegNesterov-put_pid Sometimes 1 2
> > 	Time OlegNesterov-put_pid 0.00
> > 	Hash=a3e0043ad753effa860fea37eeba0a76
> > 
> > Using WRITE_ONCE() for P0()'s store to y still allows this outcome,
> > although it does remove the "Flag data-race".
> > 
> > Using WRITE_ONCE() for both P0()'s store to y and P1()'s store to x
> > gets rid of both the "Flag data-race" and the undesired outcome:
> > 
> > 	$ herd7  -conf linux-kernel.cfg /tmp/OlegNesterov-put_pid-WO-WO.litmus 
> > 	Test OlegNesterov-put_pid-WO-WO Allowed
> > 	States 2
> > 	1:r1=0; x=1;
> > 	1:r1=1; x=2;
> > 	No
> > 	Witnesses
> > 	Positive: 0 Negative: 2
> > 	Condition exists (1:r1=1 /\ not (x=2))
> > 	Observation OlegNesterov-put_pid-WO-WO Never 0 2
> > 	Time OlegNesterov-put_pid-WO-WO 0.01
> > 	Hash=6e1643e3c5e4739b590bde0a8e8a918e
> > 
> > Here is the corresponding litmus test, in case I messed something up:
> > 
> > ------------------------------------------------------------------------
> > C OlegNesterov-put_pid-WO-WO
> > 
> > {}
> > 
> > P0(int *x, int *y)
> > {
> > 	*x = 1;
> > 	smp_mb();
> > 	WRITE_ONCE(*y, 1);
> > }
> > 
> > P1(int *x, int *y)
> > {
> > 	int r1;
> > 
> > 	r1 = READ_ONCE(*y);
> > 	if (r1)
> > 		WRITE_ONCE(*x, 2);
> > }
> > 
> > exists (1:r1=1 /\ ~x=2)
> 
> I ran the above examples too. Its a bit confusing to me why the WRITE_ONCE in
> P0() is required, and why would the READ_ONCE / WRITE_ONCE in P1() not be
> sufficient to prevent the exists condition. Shouldn't the compiler know that,
> in P0(), it should not reorder the store to y=1 before the x=1 because there
> is an explicit barrier between the 2 stores? Looks me to me like a broken
> compiler :-|. 
> 
> So I would have expected the following litmus to result in Never, but it
> doesn't with Alan's patch:
> 
> P0(int *x, int *y)
> {
> 	*x = 1;
> 	smp_mb();
> 	*y = 1;
> }
> 
> P1(int *x, int *y)
> {
> 	int r1;
> 
> 	r1 = READ_ONCE(*y);
> 	if (r1)
> 		WRITE_ONCE(*x, 2);
> }
> 
> exists (1:r1=1 /\ ~x=2)

The problem is that the compiler can turn both of P0()'s writes into reads:

P0(int *x, int *y)
{
	if (*x != 1)
		*x = 1;
	smp_mb();
	if (*y != 1)
		*y = 1;
}

These reads will not be ordered by smp_wmb(), so you have to do WRITE_ONCE()
to get an iron-clad ordering guarantee.

> > ------------------------------------------------------------------------
> > 
> > > If not, then put_pid() needs atomic_read_acquire() as it was proposed in that
> > > discussion.
> > 
> > Good point, let's try with smp_load_acquire() in P1():
> > 
> > 	$ herd7  -conf linux-kernel.cfg /tmp/OlegNesterov-put_pid-WO-sla.litmus 
> > 	Test OlegNesterov-put_pid-WO-sla Allowed
> > 	States 2
> > 	1:r1=0; x=1;
> > 	1:r1=1; x=2;
> > 	No
> > 	Witnesses
> > 	Positive: 0 Negative: 2
> > 	Condition exists (1:r1=1 /\ not (x=2))
> > 	Observation OlegNesterov-put_pid-WO-sla Never 0 2
> > 	Time OlegNesterov-put_pid-WO-sla 0.01
> > 	Hash=4fb0276eabf924793dec1970199db3a6
> > 
> > This also works.  Here is the litmus test:
> > 
> > ------------------------------------------------------------------------
> > C OlegNesterov-put_pid-WO-sla
> > 
> > {}
> > 
> > P0(int *x, int *y)
> > {
> > 	*x = 1;
> > 	smp_mb();
> > 	WRITE_ONCE(*y, 1);
> > }
> > 
> > P1(int *x, int *y)
> > {
> > 	int r1;
> > 
> > 	r1 = smp_load_acquire(y);
> > 	if (r1)
> > 		*x = 2;
> > }
> > 
> > exists (1:r1=1 /\ ~x=2)
> > ------------------------------------------------------------------------
> > 
> > Demoting P0()'s WRITE_ONCE() to a plain write while leaving P1()'s
> > smp_load_acquire() gets us a data race and allows the undesired
> > outcome:
> 
> Yeah, I think this is also what I was confused about above, is why is that
> WRITE_ONCE required in P0() because there's already an smp_mb there. Surely
> I'm missing something. ;-)

The first of P0()'s writes can be a plain write, at least assuming
sufficient synchronization to avoid the data race.  But turning the second
of P0()'s writes into a plain write is a bit riskier: That is a write of
a constant, and those really are torn in some cases on some architectures.
Like x86, for example.

> > 	$ herd7  -conf linux-kernel.cfg /tmp/OlegNesterov-put_pid-sla.litmus 
> > 	Test OlegNesterov-put_pid-sla Allowed
> > 	States 3
> > 	1:r1=0; x=1;
> > 	1:r1=1; x=1;
> > 	1:r1=1; x=2;
> > 	Ok
> > 	Witnesses
> > 	Positive: 1 Negative: 2
> > 	Flag data-race
> > 	Condition exists (1:r1=1 /\ not (x=2))
> > 	Observation OlegNesterov-put_pid-sla Sometimes 1 2
> > 	Time OlegNesterov-put_pid-sla 0.01
> > 	Hash=ec6f71f3d9f7cd6e45a874c872e3d946
> > 
> > But what if you are certain that the compiler cannot mess up your use
> > of plain C-language loads and stores?  Then simply tell LKMM that they
> > are READ_ONCE() and WRITE_ONCE(), respectively.  LKMM is admittedly
> > somewhat paranoid, but real C compilers really do tear stores of certain
> > constants on systems (like x86) that have store-immediate instructions,
> > so a bit of paranoia is not misplaced here.  ;-)
> > 
> > Plus please note that this patch to LKMM is prototype and thus subject
> > to change.
> 
> Ah I see. Appreciate if Alan can also CC me on future posting of this since
> I'm quite interested. ;-)

His last posting should be easy to find.  But please let me know if not,
as I would be happy to send it along.

							Thanx, Pau