[PATCH RFC] tools/memory-model: Fix data race detection for unordered store and load

[PATCH RFC] tools/memory-model: Fix data race detection for unordered store and load

[Alan Stern]

Currently the Linux Kernel Memory Model gives an incorrect response
for the following litmus test:

C plain-WWC

{}

P0(int *x)
{
	WRITE_ONCE(*x, 2);
}

P1(int *x, int *y)
{
	int r1;
	int r2;
	int r3;

	r1 = READ_ONCE(*x);
	if (r1 == 2) {
		smp_rmb();
		r2 = *x;
	}
	smp_rmb();
	r3 = READ_ONCE(*x);
	WRITE_ONCE(*y, r3 - 1);
}

P2(int *x, int *y)
{
	int r4;

	r4 = READ_ONCE(*y);
	if (r4 > 0)
		WRITE_ONCE(*x, 1);
}

exists (x=2 /\ 1:r2=2 /\ 2:r4=1)

The memory model says that the plain read of *x in P1 races with the
WRITE_ONCE(*x) in P2.

The problem is that we have a write W and a read R related by neither
fre or rfe, but rather W ->coe W' ->rfe R, where W' is an intermediate
write (the WRITE_ONCE() in P0).  In this situation there is no
particular ordering between W and R, so either a wr-vis link from W to
R or an rw-xbstar link from R to W would prove that the accesses
aren't concurrent.

But the LKMM only looks for a wr-vis link, which is equivalent to
assuming that W must execute before R.  This is not necessarily true
on non-multicopy-atomic systems, as the WWC pattern demonstrates.

This patch changes the LKMM to accept either a wr-vis or a reverse
rw-xbstar link as a proof of non-concurrency.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>

---

 tools/memory-model/linux-kernel.cat |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: usb-devel/tools/memory-model/linux-kernel.cat
===================================================================
--- usb-devel.orig/tools/memory-model/linux-kernel.cat
+++ usb-devel/tools/memory-model/linux-kernel.cat
@@ -197,7 +197,7 @@ empty (wr-incoh | rw-incoh | ww-incoh) a
 (* Actual races *)
 let ww-nonrace = ww-vis & ((Marked * W) | rw-xbstar) & ((W * Marked) | wr-vis)
 let ww-race = (pre-race & co) \ ww-nonrace
-let wr-race = (pre-race & (co? ; rf)) \ wr-vis
+let wr-race = (pre-race & (co? ; rf)) \ wr-vis \ rw-xbstar^-1
 let rw-race = (pre-race & fr) \ rw-xbstar
 
 flag ~empty (ww-race | wr-race | rw-race) as data-race

Re: [PATCH RFC] tools/memory-model: Fix data race detection for unordered store and load

[Paul E. McKenney]

On Fri, Sep 06, 2019 at 04:57:22PM -0400, Alan Stern wrote:
> Currently the Linux Kernel Memory Model gives an incorrect response
> for the following litmus test:
> 
> C plain-WWC
> 
> {}
> 
> P0(int *x)
> {
> 	WRITE_ONCE(*x, 2);
> }
> 
> P1(int *x, int *y)
> {
> 	int r1;
> 	int r2;
> 	int r3;
> 
> 	r1 = READ_ONCE(*x);
> 	if (r1 == 2) {
> 		smp_rmb();
> 		r2 = *x;
> 	}
> 	smp_rmb();
> 	r3 = READ_ONCE(*x);
> 	WRITE_ONCE(*y, r3 - 1);
> }
> 
> P2(int *x, int *y)
> {
> 	int r4;
> 
> 	r4 = READ_ONCE(*y);
> 	if (r4 > 0)
> 		WRITE_ONCE(*x, 1);
> }
> 
> exists (x=2 /\ 1:r2=2 /\ 2:r4=1)
> 
> The memory model says that the plain read of *x in P1 races with the
> WRITE_ONCE(*x) in P2.
> 
> The problem is that we have a write W and a read R related by neither
> fre or rfe, but rather W ->coe W' ->rfe R, where W' is an intermediate
> write (the WRITE_ONCE() in P0).  In this situation there is no
> particular ordering between W and R, so either a wr-vis link from W to
> R or an rw-xbstar link from R to W would prove that the accesses
> aren't concurrent.
> 
> But the LKMM only looks for a wr-vis link, which is equivalent to
> assuming that W must execute before R.  This is not necessarily true
> on non-multicopy-atomic systems, as the WWC pattern demonstrates.
> 
> This patch changes the LKMM to accept either a wr-vis or a reverse
> rw-xbstar link as a proof of non-concurrency.
> 
> Signed-off-by: Alan Stern <stern@rowland.harvard.edu>

Queued and pushed for review and testing, thank you very much!

						Thanx, Paul

> ---
> 
>  tools/memory-model/linux-kernel.cat |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Index: usb-devel/tools/memory-model/linux-kernel.cat
> ===================================================================
> --- usb-devel.orig/tools/memory-model/linux-kernel.cat
> +++ usb-devel/tools/memory-model/linux-kernel.cat
> @@ -197,7 +197,7 @@ empty (wr-incoh | rw-incoh | ww-incoh) a
>  (* Actual races *)
>  let ww-nonrace = ww-vis & ((Marked * W) | rw-xbstar) & ((W * Marked) | wr-vis)
>  let ww-race = (pre-race & co) \ ww-nonrace
> -let wr-race = (pre-race & (co? ; rf)) \ wr-vis
> +let wr-race = (pre-race & (co? ; rf)) \ wr-vis \ rw-xbstar^-1
>  let rw-race = (pre-race & fr) \ rw-xbstar
>  
>  flag ~empty (ww-race | wr-race | rw-race) as data-race
> 

Re: [PATCH RFC] tools/memory-model: Fix data race detection for unordered store and load

[Andrea Parri]

On Fri, Sep 06, 2019 at 04:57:22PM -0400, Alan Stern wrote:
> Currently the Linux Kernel Memory Model gives an incorrect response
> for the following litmus test:
> 
> C plain-WWC
> 
> {}
> 
> P0(int *x)
> {
> 	WRITE_ONCE(*x, 2);
> }
> 
> P1(int *x, int *y)
> {
> 	int r1;
> 	int r2;
> 	int r3;
> 
> 	r1 = READ_ONCE(*x);
> 	if (r1 == 2) {
> 		smp_rmb();
> 		r2 = *x;
> 	}
> 	smp_rmb();
> 	r3 = READ_ONCE(*x);
> 	WRITE_ONCE(*y, r3 - 1);
> }
> 
> P2(int *x, int *y)
> {
> 	int r4;
> 
> 	r4 = READ_ONCE(*y);
> 	if (r4 > 0)
> 		WRITE_ONCE(*x, 1);
> }
> 
> exists (x=2 /\ 1:r2=2 /\ 2:r4=1)
> 
> The memory model says that the plain read of *x in P1 races with the
> WRITE_ONCE(*x) in P2.
> 
> The problem is that we have a write W and a read R related by neither
> fre or rfe, but rather W ->coe W' ->rfe R, where W' is an intermediate
> write (the WRITE_ONCE() in P0).  In this situation there is no
> particular ordering between W and R, so either a wr-vis link from W to
> R or an rw-xbstar link from R to W would prove that the accesses
> aren't concurrent.
> 
> But the LKMM only looks for a wr-vis link, which is equivalent to
> assuming that W must execute before R.  This is not necessarily true
> on non-multicopy-atomic systems, as the WWC pattern demonstrates.
> 
> This patch changes the LKMM to accept either a wr-vis or a reverse
> rw-xbstar link as a proof of non-concurrency.
> 
> Signed-off-by: Alan Stern <stern@rowland.harvard.edu>

Acked-by: Andrea Parri <parri.andrea@gmail.com>

Thanks,
  Andrea


> 
> ---
> 
>  tools/memory-model/linux-kernel.cat |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Index: usb-devel/tools/memory-model/linux-kernel.cat
> ===================================================================
> --- usb-devel.orig/tools/memory-model/linux-kernel.cat
> +++ usb-devel/tools/memory-model/linux-kernel.cat
> @@ -197,7 +197,7 @@ empty (wr-incoh | rw-incoh | ww-incoh) a
>  (* Actual races *)
>  let ww-nonrace = ww-vis & ((Marked * W) | rw-xbstar) & ((W * Marked) | wr-vis)
>  let ww-race = (pre-race & co) \ ww-nonrace
> -let wr-race = (pre-race & (co? ; rf)) \ wr-vis
> +let wr-race = (pre-race & (co? ; rf)) \ wr-vis \ rw-xbstar^-1
>  let rw-race = (pre-race & fr) \ rw-xbstar
>  
>  flag ~empty (ww-race | wr-race | rw-race) as data-race
> 

Re: [PATCH RFC] tools/memory-model: Fix data race detection for unordered store and load

[Paul E. McKenney]

On Tue, Sep 17, 2019 at 01:39:59PM +0200, Andrea Parri wrote:
> On Fri, Sep 06, 2019 at 04:57:22PM -0400, Alan Stern wrote:
> > Currently the Linux Kernel Memory Model gives an incorrect response
> > for the following litmus test:
> > 
> > C plain-WWC
> > 
> > {}
> > 
> > P0(int *x)
> > {
> > 	WRITE_ONCE(*x, 2);
> > }
> > 
> > P1(int *x, int *y)
> > {
> > 	int r1;
> > 	int r2;
> > 	int r3;
> > 
> > 	r1 = READ_ONCE(*x);
> > 	if (r1 == 2) {
> > 		smp_rmb();
> > 		r2 = *x;
> > 	}
> > 	smp_rmb();
> > 	r3 = READ_ONCE(*x);
> > 	WRITE_ONCE(*y, r3 - 1);
> > }
> > 
> > P2(int *x, int *y)
> > {
> > 	int r4;
> > 
> > 	r4 = READ_ONCE(*y);
> > 	if (r4 > 0)
> > 		WRITE_ONCE(*x, 1);
> > }
> > 
> > exists (x=2 /\ 1:r2=2 /\ 2:r4=1)
> > 
> > The memory model says that the plain read of *x in P1 races with the
> > WRITE_ONCE(*x) in P2.
> > 
> > The problem is that we have a write W and a read R related by neither
> > fre or rfe, but rather W ->coe W' ->rfe R, where W' is an intermediate
> > write (the WRITE_ONCE() in P0).  In this situation there is no
> > particular ordering between W and R, so either a wr-vis link from W to
> > R or an rw-xbstar link from R to W would prove that the accesses
> > aren't concurrent.
> > 
> > But the LKMM only looks for a wr-vis link, which is equivalent to
> > assuming that W must execute before R.  This is not necessarily true
> > on non-multicopy-atomic systems, as the WWC pattern demonstrates.
> > 
> > This patch changes the LKMM to accept either a wr-vis or a reverse
> > rw-xbstar link as a proof of non-concurrency.
> > 
> > Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
> 
> Acked-by: Andrea Parri <parri.andrea@gmail.com>

Applied, thank you both!

							Thanx, Paul