On Tue, May 29, 2018 at 02:35:34PM -0400, Alan Stern wrote: > On Mon, 28 May 2018, Paul E. McKenney wrote: > > > Hello! > > > > The litmus test below is a first attempt to model Roman's rcu-rr > > round-robin RCU-protected linked list. His test code, which includes > > the algorithm under test, may be found here: > > > > https://github.com/rouming/rcu-rr/blob/master/rcu-rr.c > > > > The P0() process below roughly corresponds to remove_conn_from_arr(), > > with litmus-test variable "c" standing in for the per-CPU ppcpu_con. > > Similarly, P1() roughly corresponds to get_next_conn_rr(). It claims > > that the algorithm is safe, and also claims that it becomes unsafe if > > either synchronize_rcu() is removed. > > This algorithm (the one in the litmus test; I haven't looked at Roman's > code) does seem valid. In addition to removing either > synchronize_rcu(), interchanging the order of the stores in P0 (c > first, then w) would also invalidate it. > > This is a little unusual in that c is written by more than one thread > with no protection. It works because the writes are all stores of a > single pointer. > > Why does the litmus test use smp_store_release() in three places? > There doesn't seem to be any need; WRITE_ONCE() would be sufficient. Because the algorithm did. A bit of a stretch for kfree, but... ;-) Let's try removing them, please see below. > Alan > > > Does this in fact realistically model Roman's algorithm? Either way, > > is there a better approach? > > > > Thanx, Paul > > > > ------------------------------------------------------------------------ > > > > C C-RomanPenyaev-list-rcu-rr > > > > { > > int *z=1; (* List: v->w->x->y->z. Noncircular, but long enough. *) > > int *y=z; > > int *x=y; > > int *w=x; > > int *v=w; (* List head is v. *) > > int *c=w; (* Cache, emulating ppcpu_con. *) > > } > > > > P0(int *c, int *v, int *w, int *x, int *y) > > { > > rcu_assign_pointer(*w, y); /* Remove x from list. */ No change when converting this to WRITE_ONCE(); > > synchronize_rcu(); > > r1 = READ_ONCE(*c); > > if (r1 == x) { > > WRITE_ONCE(*c, 0); /* Invalidate cache. */ > > synchronize_rcu(); > > } > > smp_store_release(x, 0); /* Emulate kfree(x). */ Converting this one to WRITE_ONCE() does have an effect: Test C-RomanPenyaev-list-rcu-rr Allowed States 8 0:r1=0; 1:r1=w; 1:r2=x; 1:r3=x; 1:r4=0; c=0; v=w; w=y; x=0; y=z; 0:r1=w; 1:r1=w; 1:r2=y; 1:r3=y; 1:r4=z; c=z; v=w; w=y; x=0; y=z; 0:r1=x; 1:r1=w; 1:r2=x; 1:r3=w; 1:r4=y; c=y; v=w; w=y; x=0; y=z; 0:r1=x; 1:r1=w; 1:r2=x; 1:r3=x; 1:r4=y; c=0; v=w; w=y; x=0; y=z; 0:r1=x; 1:r1=w; 1:r2=x; 1:r3=x; 1:r4=y; c=y; v=w; w=y; x=0; y=z; 0:r1=y; 1:r1=w; 1:r2=x; 1:r3=x; 1:r4=y; c=y; v=w; w=y; x=0; y=z; 0:r1=y; 1:r1=w; 1:r2=y; 1:r3=y; 1:r4=z; c=z; v=w; w=y; x=0; y=z; 0:r1=z; 1:r1=w; 1:r2=y; 1:r3=y; 1:r4=z; c=z; v=w; w=y; x=0; y=z; Ok Witnesses Positive: 1 Negative: 7 Condition exists (1:r1=0 \/ 1:r2=0 \/ 1:r3=0 \/ 1:r4=0) Observation C-RomanPenyaev-list-rcu-rr Sometimes 1 7 Time C-RomanPenyaev-list-rcu-rr 0.40 Hash=2ec66290a6622117b9877436950e6a08 Maybe reordered with READ_ONCE(*c) when r1 != x? > > } > > > > P1(int *c, int *v) > > { > > rcu_read_lock(); > > r1 = READ_ONCE(*c); /* Pick up cache. */ > > if (r1 == 0) { > > r1 = READ_ONCE(*v); /* Cache empty, start from head. */ > > } > > r2 = rcu_dereference(*r1); /* Advance to next element. */ > > smp_store_release(c, r2); /* Update cache. */ > > rcu_read_unlock(); > > > > /* And repeat. */ > > rcu_read_lock(); > > r3 = READ_ONCE(*c); > > if (r3 == 0) { > > r3 = READ_ONCE(*v); > > } > > r4 = rcu_dereference(*r3); > > smp_store_release(c, r4); Converting this to WRITE_ONCE() has no effect. > > rcu_read_unlock(); > > } > > > > locations [0:r1; 1:r1; 1:r3; c; v; w; x; y] > > exists (1:r1=0 \/ 1:r2=0 \/ 1:r3=0 \/ 1:r4=0) (* Better not be freed!!! *) > Thanx, Paul
