On Sat, Oct 03, 2020 at 12:11:59PM -0400, joel@xxxxxxxxxxxxxxxxx wrote: > On Sat, Oct 03, 2020 at 12:08:46PM -0400, joel@xxxxxxxxxxxxxxxxx wrote: > [...] > > static void code0(struct v_struct* v,spinlock_t* l,int* out_0_r1) { > > > > struct v_struct *r1; /* to_free */ > > > > r1 = NULL; > > spin_lock(l); > > if (!smp_load_acquire(&v->b)) > > r1 = v; > > v->a = 0; > > spin_unlock(l); > > > > *out_0_r1 = !!r1; > > } > > > > static void code1(struct v_struct* v,spinlock_t* l,int* out_1_r1) { > > > > struct v_struct *r1; /* to_free */ > > > > r1 = v; > > if (READ_ONCE(v->a)) { > > spin_lock(l); > > if (v->a) > > r1 = NULL; > > smp_store_release(&v->b, 0); > > spin_unlock(l); > > } > > > > *out_1_r1 = !!r1; > > } > > > > Results on both arm64 and x86: > > > > Histogram (2 states) > > 19080852:>0:r1=1; 1:r1=0; > > 20919148:>0:r1=0; 1:r1=1; > > No > > > > Witnesses > > Positive: 0, Negative: 40000000 > > Condition exists (0:r1=1 /\ 1:r1=1) is NOT validated > > Hash=4a8c15603ffb5ab464195ea39ccd6382 > > Observation AL+test Never 0 40000000 > > Time AL+test 6.24 > > > > I guess I could do an alloc and free of v_struct. However, I just checked for > > whether the to_free in Al's example could ever be NULL for both threads. > > Sorry, here I meant "ever be non-NULL". > > So basically I was trying to experimentally confirm that to_free could never > be non-NULL in both code0 and code1 threads. Thank you for running these! In conjunction with Alan's analysis, this seems quite convincing. ;-) Thanx, Paul