On 05/11/2018 07:20 PM, 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> > --- > v2 changes: Corrections as suggested by Randy. > > 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..533bc1087371 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. > + * > + * We do this with a single addition and masking. > + * 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
it's > + * add (2+1) and mask with ~1. Lets see why with an example: Let's I.e., Let's not be so casual with (dropping) apostrophes. But v3 can wait for other comments. :) > + * > + * 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. > + */ > static inline unsigned long rcu_seq_snap(unsigned long *sp) > { > unsigned long s; > -- ~Randy