Messages in this thread | | | Date | Wed, 27 Jun 2018 10:54:36 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 16/27] rcu: Add comment documenting how rcu_seq_snap works |
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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
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