Commit 3636d8d1 authored by Frederic Weisbecker's avatar Frederic Weisbecker Committed by Joel Fernandes (Google)
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rcu: Further comment and explain the state space of GP sequences 



The state space of the GP sequence number isn't documented and the
definitions of its special values are scattered.  This commit therefore
gathers some common knowledge near the grace-period sequence-number
definitions.

Signed-off-by: default avatarFrederic Weisbecker <frederic@kernel.org>
Reviewed-by: default avatarJoel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
Signed-off-by: default avatarJoel Fernandes (Google) <joel@joelfernandes.org>
parent fe15c26e

kernel/rcu/rcu.h

+37 −0
Original line number Diff line number Diff line
@@ -14,6 +14,43 @@ 


/*

 * Grace-period counter management.

 *

 * The two least significant bits contain the control flags.

 * The most significant bits contain the grace-period sequence counter.

 *

 * When both control flags are zero, no grace period is in progress.

 * When either bit is non-zero, a grace period has started and is in

 * progress. When the grace period completes, the control flags are reset

 * to 0 and the grace-period sequence counter is incremented.

 *

 * However some specific RCU usages make use of custom values.

 *

 * SRCU special control values:

 *

 *	SRCU_SNP_INIT_SEQ	:	Invalid/init value set when SRCU node

 *					is initialized.

 *

 *	SRCU_STATE_IDLE		:	No SRCU gp is in progress

 *

 *	SRCU_STATE_SCAN1	:	State set by rcu_seq_start(). Indicates

 *					we are scanning the readers on the slot

 *					defined as inactive (there might well

 *					be pending readers that will use that

 *					index, but their number is bounded).

 *

 *	SRCU_STATE_SCAN2	:	State set manually via rcu_seq_set_state()

 *					Indicates we are flipping the readers

 *					index and then scanning the readers on the

 *					slot newly designated as inactive (again,

 *					the number of pending readers that will use

 *					this inactive index is bounded).

 *

 * RCU polled GP special control value:

 *

 *	RCU_GET_STATE_COMPLETED :	State value indicating an already-completed

 *					polled GP has completed.  This value covers

 *					both the state and the counter of the

 *					grace-period sequence number.

 */



#define RCU_SEQ_CTR_SHIFT	2