On Mon, May 14, 2018 at 10:38:16AM -0700, Paul E. McKenney wrote: > On Sun, May 13, 2018 at 08:15:34PM -0700, Joel Fernandes (Google) wrote: > > rcu_seq_snap may be tricky for someone looking at it for the first time. > > Lets document how it works with an example to make it easier. > > > > Signed-off-by: Joel Fernandes (Google) <j...@joelfernandes.org> > > --- > > kernel/rcu/rcu.h | 24 +++++++++++++++++++++++- > > 1 file changed, 23 insertions(+), 1 deletion(-) > > > > diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h > > index 003671825d62..fc3170914ac7 100644 > > --- a/kernel/rcu/rcu.h > > +++ b/kernel/rcu/rcu.h > > @@ -91,7 +91,29 @@ static inline void rcu_seq_end(unsigned long *sp) > > WRITE_ONCE(*sp, rcu_seq_endval(sp)); > > } > > > > -/* Take a snapshot of the update side's sequence number. */ > > +/* > > + * Take a snapshot of the update side's sequence number. > > + * > > + * This function predicts what the grace period number will be the next > > + * time an RCU callback will be executed, given the current grace period's > > + * number. This can be gp+1 if RCU is idle, or gp+2 if a grace period is > > + * already in progress. > > How about something like this? > > 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.
This makes sense too, but do you disagree with what I said? I was kind of thinking of snap along the lines of how the previous code worked. Where you were calling rcu_cbs_completed() or a function with a similar name. Now we call _snap. So basically I connected these 2 facts together to mean that rcu_seq_snap also does that same thing as rcu_cbs_completed - which is basically it gives the "next GP" where existing callbacks have already run and new callbacks will run at the end of this "next GP". > > + * > > + * We do this with a single addition and masking. > > Please either fold this sentence into rest of the paragraph or add a > blank line after it. > > > + * For example, if RCU_SEQ_STATE_MASK=1 and the least significant bit > > (LSB) of > > + * the seq is used to track if a GP is in progress or not, its sufficient > > if we > > + * add (2+1) and mask with ~1. Let's see why with an example: > > + * > > + * Say the current seq is 6 which is 0b110 (gp is 3 and state bit is 0). > > + * To get the next GP number, we have to at least add 0b10 to this (0x1 << > > 1) > > + * to account for the state bit. However, if the current seq is 7 (gp is 3 > > and > > + * state bit is 1), then it means the current grace period is already in > > + * progress so the next time the callback will run is at the end of grace > > + * period number gp+2. To account for the extra +1, we just overflow the > > LSB by > > + * adding another 0x1 and masking with ~0x1. In case no GP was in progress > > (RCU > > + * is idle), then the addition of the extra 0x1 and masking will have no > > + * effect. This is calculated as below. > > + */ > > Having the explicit numbers is good, but please use RCU_SEQ_STATE_MASK=3, > since that is the current value. One alternative (or perhaps addition) > is to have a short table of numbers showing the mapping from *sp to the > return value. (I started from such a table when writing this function, > for whatever that is worth.) Ok I'll try to give a better example above. thanks, Also just to let you know, thanks so much for elaborately providing an example on the other thread where we are discussing the rcu_seq_done check. I will take some time to trace this down and see if I can zero in on the same understanding as yours. I get why we use rcu_seq_snap there in rcu_start_this_gp but the way it its used is 'c' is the requested GP obtained from _snap, and we are comparing that with the existing rnp->gp_seq in rcu_seq_done. When that rnp->gp_seq reaches 'c', it only means rnp->gp_seq is done, it doesn't tell us if 'c' is done which is what we were trying to check in that loop... that's why I felt that check wasn't correct - that's my (most likely wrong) take on the matter, and I'll get back once I trace this a bit more hopefully today :-P thanks! - Joel
