[PATCH] rcu: Add comment documenting how rcu_seq_snap works

[Randy Dunlap]

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) <joel@xxxxxxxxxxxxxxxxx>
> ---
>  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