On Mon, Jun 03, 2019 at 01:26:26PM +0800, Herbert Xu wrote: > On Sun, Jun 02, 2019 at 08:47:07PM -0700, Paul E. McKenney wrote: > > > > 1. These guarantees are of full memory barriers, -not- compiler > > barriers. > > What I'm saying is that wherever they are, they must come with > compiler barriers. I'm not aware of any synchronisation mechanism > in the kernel that gives a memory barrier without a compiler barrier.
Yes, if a given synchronization mechanism requires that memory references need to be ordered, both the compiler and the CPU must maintain that ordering. > > 2. These rules don't say exactly where these full memory barriers > > go. SRCU is at one extreme, placing those full barriers in > > srcu_read_lock() and srcu_read_unlock(), and !PREEMPT Tree RCU > > at the other, placing these barriers entirely within the callback > > queueing/invocation, grace-period computation, and the scheduler. > > Preemptible Tree RCU is in the middle, with rcu_read_unlock() > > sometimes including a full memory barrier, but other times with > > the full memory barrier being confined as it is with !PREEMPT > > Tree RCU. > > The rules do say that the (full) memory barrier must precede any > RCU read-side that occur after the synchronize_rcu and after the > end of any RCU read-side that occur before the synchronize_rcu. > > All I'm arguing is that wherever that full mb is, as long as it > also carries with it a barrier() (which it must do if it's done > using an existing kernel mb/locking primitive), then we're fine. Fair enough, and smp_mb() does provide what is needed. > > Interleaving and inserting full memory barriers as per the rules above: > > > > CPU1: WRITE_ONCE(a, 1) > > CPU1: synchronize_rcu > > /* Could put a full memory barrier here, but it wouldn't help. */ > > CPU1: smp_mb(); > CPU2: smp_mb(); What is CPU2's smp_mb() ordering? In other words, what comment would you put on each of the above smp_mb() calls? > Let's put them in because I think they are critical. smp_mb() also > carries with it a barrier(). Again, agreed, smp_mb() implies barrier(). > > CPU2: rcu_read_lock(); > > CPU1: b = 2; > > CPU2: if (READ_ONCE(a) == 0) > > CPU2: if (b != 1) /* Weakly ordered CPU moved this up! */ > > CPU2: b = 1; > > CPU2: rcu_read_unlock > > > > In fact, CPU2's load from b might be moved up to race with CPU1's store, > > which (I believe) is why the model complains in this case. > > Let's put aside my doubt over how we're even allowing a compiler > to turn > > b = 1 > > into > > if (b != 1) > b = 1 > > Since you seem to be assuming that (a == 0) is true in this case > (as the assignment b = 1 is carried out), then because of the > presence of the full memory barrier, the RCU read-side section > must have started prior to the synchronize_rcu. This means that > synchronize_rcu is not allowed to return until at least the end > of the grace period, or at least until the end of rcu_read_unlock. > > So it actually should be: > > CPU1: WRITE_ONCE(a, 1) > CPU1: synchronize_rcu called > /* Could put a full memory barrier here, but it wouldn't help. */ > > CPU1: smp_mb(); > CPU2: smp_mb(); > > CPU2: grace period starts > ...time passes... > CPU2: rcu_read_lock(); > CPU2: if (READ_ONCE(a) == 0) > CPU2: if (b != 1) /* Weakly ordered CPU moved this up! */ > CPU2: b = 1; > CPU2: rcu_read_unlock > ...time passes... > CPU2: grace period ends > > /* This full memory barrier is also guaranteed by RCU. */ > CPU2: smp_mb(); But in this case, given that there are no more statements for CPU2, what is this smp_mb() ordering? Thanx, Paul > CPU1 synchronize_rcu returns > CPU1: b = 2; > > Cheers, > -- > Email: Herbert Xu <herb...@gondor.apana.org.au> > Home Page: http://gondor.apana.org.au/~herbert/ > PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt >