On Tue, Jun 26, 2018 at 09:39:13PM -0700, Joel Fernandes wrote:
> On Tue, Jun 26, 2018 at 07:30:55PM +0200, Peter Zijlstra wrote:
> > On Mon, Jun 25, 2018 at 05:35:02PM -0700, Paul E. McKenney wrote:
> > > +/*
> > > + * rcu_seq_snap - Take a snapshot of the update side's sequence number.
> > > + *
> > > + * This function returns the earliest value of the grace-period sequence
> > > number
> > > + * that will indicate that a full grace period has elapsed since the
> > > current
> > > + * time. Once the grace-period sequence number has reached this value,
> > > it will
> > > + * be safe to invoke all callbacks that have been registered prior to the
> > > + * current time. This value is the current grace-period number plus two
> > > to the
> > > + * power of the number of low-order bits reserved for state, then
> > > rounded up to
> > > + * the next value in which the state bits are all zero.
> >
> > If you complete that by saying _why_ you need to round up there, then
> > the below verbiage is completely redundant.
> >
> > > + * In the current design, RCU_SEQ_STATE_MASK=3 and the least significant
> > > bit of
> > > + * the seq is used to track if a GP is in progress or not. Given this,
> > > it is
> > > + * sufficient if we add (6+1) and mask with ~3 to get the next GP. Let's
> > > see
> > > + * why with an example:
> > > + *
> > > + * Say the current seq is 12 which is 0b1100 (GP is 3 and state bits are
> > > 0b00).
> > > + * To get to the next GP number of 4, we have to add 0b100 to this (0x1
> > > << 2)
> > > + * to account for the shift due to 2 state bits. Now, if the current seq
> > > is
> > > + * 13 (GP is 3 and state bits are 0b01), then it means the current grace
> > > period
> > > + * is already in progress so the next GP that a future call back will be
> > > queued
> > > + * to run at is GP+2 = 5, not 4. To account for the extra +1, we just
> > > overflow
> > > + * the 2 lower bits by adding 0b11. In case the lower bit was set, the
> > > overflow
> > > + * will cause the extra +1 to the GP, along with the usual +1 explained
> > > before.
> > > + * This gives us GP+2. Finally we mask the lower to bits by ~0x3 in case
> > > the
> > > + * overflow didn't occur. This masking is needed because in case RCU was
> > > idle
> > > + * (no GP in progress so lower 2 bits are 0b00), then the overflow of
> > > the lower
> > > + * 2 state bits wouldn't occur, so we mask to zero out those lower 2
> > > bits.
> > > + *
> > > + * In other words, the next seq can be obtained by (0b11 + 0b100) &
> > > (~0b11)
> > > + * which can be generalized to:
> > > + * seq + (RCU_SEQ_STATE_MASK + (RCU_SEQ_STATE_MASK + 1)) &
> > > (~RCU_SEQ_STATE_MASK)
> > > + */
> >
> > Is the below not much simpler:
> >
> > > static inline unsigned long rcu_seq_snap(unsigned long *sp)
> > > {
> > > unsigned long s;
> >
> > s = smp_load_aquire(sp);
> >
> > /* Add one GP */
> > s += 1 << RCU_SEQ_CTR_SHIFT;
> >
> > /* Complete any pending state by rounding up */
>
> I would suggest this comment be changed to "Add another GP if there was a
> pending state".
>
> > s = __ALIGN_MASK(s, RCU_SEQ_STATE_MASK);
> >
>
> I agree with Peter's suggestions for both the verbiage reduction in the
> comments in the header, as the new code he is proposing is more
> self-documenting. I believe I proposed a big comment just because the code
> wasn't self-documenting or obvious previously so needed an explanation.
>
> How would you like to proceed? Let me know what you guys decide, I am really
> Ok with anything. If you guys agree, should I write a follow-up patch with
> Peter's suggestion that applies on top of this one? Or do we want to drop
> this one in favor of Peter's suggestion?
Shortening the comment would be good, so please do that.
I cannot say that I am much of a fan of the suggested change to the
computation, but I don't feel all that strongly about it. If the two
of you agree on a formulation and get at least one other RCU maintainer
or reviewer to agree as well, I will take the change.
> I guess we also have to conclude the other part about using memory barriers,
> but I think that should be a separate patch.
It definitely should not be part of this patch. ;-)
Thanx, Paul
