On 05/11/2018 05:33 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> > --- > kernel/rcu/rcu.h | 23 ++++++++++++++++++++++- > 1 file changed, 22 insertions(+), 1 deletion(-) > > diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h > index 003671825d62..004ace3d22c2 100644 > --- a/kernel/rcu/rcu.h > +++ b/kernel/rcu/rcu.h > @@ -91,7 +91,28 @@ 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 > + * add (2+1) and mask with ~1. Lets see why with an example: > + * > + * Say the current seq is 6 which is 0x110 (gp is 3 and state bit is 0).
0b110 or 0x6 > + * To get the next GP number, we have to atleast add 0x10 to this (0x1 << 1) > to at least add 0b10 > + * account for the state bit. However, if the current seq is 7 (GP num is 3 > + * and state bit is 1), then it means the current grace period is already > + * in progress so the next the callback will run is at gp+2. To account for so the next time? the callback will run > + * the extra +1, we just overflow the LSB by adding another 0x1 and masking > + * with ~0x1. Incase no GP was in progress (RCU is idle), then the adding In case > + * by 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