Messages in this thread | | | Date | Sat, 28 Jan 2023 17:59:52 -0500 | From | Alan Stern <> | Subject | Re: [PATCH v2 2/2] tools/memory-model: Make ppo a subrelation of po |
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On Sat, Jan 28, 2023 at 11:14:17PM +0100, Andrea Parri wrote: > > Evidently the plain-coherence check rules out x=1 at the > > end, because when I relax that check, x=1 becomes a possible result. > > Furthermore, the graphical output confirms that this execution has a > > ww-incoh edge from Wx=2 to Wx=1. But there is no ww-vis edge from Wx=1 > > to Wx=2! How can this be possible? It seems like a bug in herd7. > > By default, herd7 performs some edges removal when generating the > graphical outputs. The option -showraw can be useful to increase > the "verbosity", for example, > > [with "exists (x=2)", output in /tmp/T.dot] > $ herd7 -conf linux-kernel.cfg T.litmus -show prop -o /tmp -skipchecks plain-coherence -doshow ww-vis -showraw ww-vis
Okay, thanks, that helps a lot.
So here's what we've got. The litmus test:
C hb-and-int {}
P0(int *x, int *y) { *x = 1; smp_store_release(y, 1); }
P1(int *x, int *y, int *dx, int *dy, spinlock_t *l) { spin_lock(l); int r1 = READ_ONCE(*dy); if (r1==1) spin_unlock(l);
int r0 = smp_load_acquire(y); if (r0 == 1) { WRITE_ONCE(*dx,1); } }
P2(int *dx, int *dy) { WRITE_ONCE(*dy,READ_ONCE(*dx)); }
P3(int *x, spinlock_t *l) { spin_lock(l); smp_mb__after_unlock_lock(); *x = 2; }
exists (x=2)
The reason why Wx=1 ->ww-vis Wx=2:
0:Wx=1 ->po-rel 0:Wy=1 and po-rel < fence < ww-post-bounded.
0:Wy=1 ->rfe 1:Ry=1 ->(hb* & int) 1:Rdy=1 and (rfe ; hb* & int) <= (rfe ; xbstar & int) <= vis.
1:Rdy=1 ->po 1:unlock ->rfe 3:lock ->po 3:Wx=2 so 1:Rdy=1 ->po-unlock-lock-po 3:Wx=2 and po-unlock-lock-po <= mb <= fence <= w-pre-bounded.
Finally, w-post-bounded ; vis ; w-pre-bounded <= ww-vis.
This explains why the memory model says there isn't a data race. This doesn't use the smp_mb__after_unlock_lock at all.
Alan
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