  `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.hindex 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`   