Messages in this thread Patch in this message | | | Date | Fri, 11 May 2018 17:33:06 -0700 | From | "Joel Fernandes (Google)" <> | Subject | [PATCH] rcu: Add comment documenting how rcu_seq_snap works |
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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@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). + * To get the next GP number, we have to atleast add 0x10 to this (0x1 << 1) to + * 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 + * 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 + * 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; -- 2.17.0.441.gb46fe60e1d-goog
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