Re: [PATCH] Convert struct pid count to refcount_t

[Paul E. McKenney]

On Mon, Apr 01, 2019 at 05:11:39PM -0400, Joel Fernandes wrote:
> Thanks a lot Alan and Paul for the replies. I replied inline.
> 
> On Sun, Mar 31, 2019 at 02:55:31PM -0700, Paul E. McKenney wrote:
> > 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@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)
> > 
> > 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.
> 
> But the snippet above has smp_mb() which does order writes, even for the
> plain accesses.

True, but that code has a data race, namely P0()'s plain write to y
races with P1()'s READ_ONCE().  Data races give the compiler absolutely
astonishing levels of freedom to rearrange your code.  So, if you
as a developer or maintainer choose to have data races, it is your
responsibility to ensure that the compiler cannot mess you up.  So what
you should do in that case is to list the transformed code the compiler's
optimizations can produce and feed the corresponding litmus tests to LKMM,
using READ_ONCE() and WRITE_ONCE() for the post-optimization accesses.

> > > > ------------------------------------------------------------------------
> > > > 
> > > > > 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.
> 
> I understand. Are we talking about load/store tearing being the issue here?
> Even under load/store tearing, I feel the program will produce correct
> results because r1 is either 0 or 1 (a single bit cannot be torn).

The compiler can (at least in theory) also transform *y = 1 as follows:

	*y = r1; /* Use *y as temporary storage. */
	do_something();
	r1 = *y;
	*y = 1;

Here r1 is some register and do_something() is an inline function visible
to the compiler (hence not implying barrier() upon call and return).

I don't know of any compilers that actually do this, but for me use
of WRITE_ONCE() is a small price to pay to prevent all past, present,
and future compiler from ever destroying^W optimizing my code in this way.

In your own code, if you dislike WRITE_ONCE() so much that you are willing
to commit to (now and forever!!!) checking all applicable versions of
compilers to make sure that they don't do this, well and good, knock
yourself out.  But it is your responsibility to do that checking, and you
can attest to LKMM that you have done so by giving LKMM litmus tests that
substitute WRITE_ONCE() for that plain C-language assignment statement.

> Further, from the herd simulator output (below), according to the "States",
> r1==1 means P1() AFAICS would have already finished the the read and set the
> r1 register to 1.  Then I am wondering why it couldn't take the branch to set
> *x to 2.  According to herd, r1 == 1 AND x == 1 is a perfectly valid state
> for the below program. I still couldn't see in my mind how for the below
> program, this is possible - in terms of compiler optimizations or other kinds
> of ordering. Because there is a smp_mb() between the 2 plain writes in P0()
> and P1() did establish that r1 is 1 in the positive case. :-/.  I am surely
> missing something :-)
> 
> ---8<-----------------------
> C Joel-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)
> 		WRITE_ONCE(*x, 2);
> }
> 
> exists (1:r1=1 /\ ~x=2)
> 
> ---8<-----------------------
> Output:
> 
> Test Joel-put_pid Allowed
> States 3
> 1:r1=0; x=1;
> 1:r1=1; x=1;	<-- Can't figure out why r1=1 and x != 2 here.

I must defer to Alan on this, but my guess is that this is due to
the fact that there is a data race.

							Thanx, Paul

> 1:r1=1; x=2;
> Ok
> Witnesses
> Positive: 1 Negative: 2
> Flag data-race
> Condition exists (1:r1=1 /\ not (x=2))
> Observation Joel-put_pid Sometimes 1 2
> Time OlegNesterov-put_pid-WO-WO 0.01
> Hash=c7bdd50418d42779b7c10ba9128369df
> 
> > > > 	$ 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.
> 
> I found it and I'm going through it. Thanks!
>  
>  - Joel
>