Re: [PATCH] Convert struct pid count to refcount_t

[Joel Fernandes]

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@xxxxxxxxxxxxxxxxxxxx
> 
> 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)

> ------------------------------------------------------------------------
> 
> > 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. ;-)

> 
> 	$ 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. ;-)

thanks,

 - Joel

> 							Thanx, Paul
>