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Date: Fri, 6 Oct 2017 12:18:22 -0700 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Peter Zijlstra <peterz@...radead.org> Cc: linux-kernel@...r.kernel.org, mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com, akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com, josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com, oleg@...hat.com Subject: Re: [PATCH tip/core/rcu 1/9] rcu: Provide GP ordering in face of migrations and delays On Fri, Oct 06, 2017 at 11:07:23AM +0200, Peter Zijlstra wrote: > On Thu, Oct 05, 2017 at 11:22:04AM -0700, Paul E. McKenney wrote: > > Hmmm... Here is what I was worried about: > > > > C C-PaulEMcKenney-W+RWC4+2017-10-05 > > > > { > > } > > > > P0(int *a, int *x) > > { > > WRITE_ONCE(*a, 1); > > smp_mb(); /* Lock acquisition for rcu_node ->lock. */ > > WRITE_ONCE(*x, 1); > > } > > > > P1(int *x, int *y, spinlock_t *l) > > { > > r3 = READ_ONCE(*x); > > smp_mb(); /* Lock acquisition for rcu_node ->lock. */ > > spin_lock(l); /* Locking in complete(). */ > > WRITE_ONCE(*y, 1); > > spin_unlock(l); > > } > > > > P2(int *y, int *b, spinlock_t *l) > > { > > spin_lock(l); /* Locking in wait_for_completion. */ > > r4 = READ_ONCE(*y); > > spin_unlock(l); > > r1 = READ_ONCE(*b); > > } > > > > P3(int *b, int *a) > > { > > WRITE_ONCE(*b, 1); > > smp_mb(); > > r2 = READ_ONCE(*a); > > } > > > > exists (1:r3=1 /\ 2:r4=1 /\ 2:r1=0 /\ 3:r2=0) > > /me goes and install this herd thing again.. I'm sure I had it running > _somewhere_.. A well. > > C C-PaulEMcKenney-W+RWC4+2017-10-05 > > { > } > > P0(int *a, int *x) > { > WRITE_ONCE(*a, 1); > smp_mb(); /* Lock acquisition for rcu_node ->lock. */ > WRITE_ONCE(*x, 1); > } > > P1(int *x, int *y) > { > r3 = READ_ONCE(*x); > smp_mb(); /* Lock acquisition for rcu_node ->lock. */ > smp_store_release(y, 1); > } > > P2(int *y, int *b) > { > r4 = smp_load_acquire(y); > r1 = READ_ONCE(*b); > } > > P3(int *b, int *a) > { > WRITE_ONCE(*b, 1); > smp_mb(); > r2 = READ_ONCE(*a); > } > > exists (1:r3=1 /\ 2:r4=1 /\ 2:r1=0 /\ 3:r2=0) > > > Is what I was thinking of, I think that is the minimal ordering > complete()/wait_for_completion() need to provide. OK, I will bite... What do the smp_store_release() and the smp_load_acquire() correspond to? I see just plain locking in wait_for_completion() and complete(). > (also, that r# numbering confuses the hell out of me, its not related to > P nor to the variables) Yeah, it is random, sorry!!! > Test C-PaulEMcKenney-W+RWC4+2017-10-05 Allowed > States 15 > 1:r3=0; 2:r1=0; 2:r4=0; 3:r2=0; > 1:r3=0; 2:r1=0; 2:r4=0; 3:r2=1; > 1:r3=0; 2:r1=0; 2:r4=1; 3:r2=0; > 1:r3=0; 2:r1=0; 2:r4=1; 3:r2=1; > 1:r3=0; 2:r1=1; 2:r4=0; 3:r2=0; > 1:r3=0; 2:r1=1; 2:r4=0; 3:r2=1; > 1:r3=0; 2:r1=1; 2:r4=1; 3:r2=0; > 1:r3=0; 2:r1=1; 2:r4=1; 3:r2=1; > 1:r3=1; 2:r1=0; 2:r4=0; 3:r2=0; > 1:r3=1; 2:r1=0; 2:r4=0; 3:r2=1; > 1:r3=1; 2:r1=0; 2:r4=1; 3:r2=1; > 1:r3=1; 2:r1=1; 2:r4=0; 3:r2=0; > 1:r3=1; 2:r1=1; 2:r4=0; 3:r2=1; > 1:r3=1; 2:r1=1; 2:r4=1; 3:r2=0; > 1:r3=1; 2:r1=1; 2:r4=1; 3:r2=1; > No > Witnesses > Positive: 0 Negative: 15 > Condition exists (1:r3=1 /\ 2:r4=1 /\ 2:r1=0 /\ 3:r2=0) > Observation C-PaulEMcKenney-W+RWC4+2017-10-05 Never 0 15 > Time C-PaulEMcKenney-W+RWC4+2017-10-05 0.04 > Hash=f7f8ad6eab33e90718a394bcb021557d But yes, looking closer, this corresponds to the rule of thumb about non-rf relations and full memory barriers. We have two non-rf relations (P2->P3 and P3->P0), so we need two full barriers, one each between the non-rf relations. So I dropped that patch yesterday. The main thing I was missing was that there is no ordering-free fastpath in wait_for_completion() and complete(): Each unconditionally acquires the lock. So the smp_mb() that I was trying to add doesn't need to be there. Thanx, Paul
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