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rcu: Add comment documenting how rcu_seq_snap works

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rcu_seq_snap may be tricky to decipher. Lets document how it works with
an example to make it easier.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Shrink comment as suggested by Peter Zijlstra. ]
This commit is contained in:
Joel Fernandes (Google) 2018-05-22 23:38:13 -07:00 committed by Paul E. McKenney
parent b06ae25a1e
commit 0d805a70a6
1 changed files with 11 additions and 1 deletions
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12
kernel/rcu/rcu.h
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@ -91,7 +91,17 @@ 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. */
/*
* rcu_seq_snap - Take a snapshot of the update side's sequence number.
*
* This function returns the earliest value of the grace-period sequence number
* that will indicate that a full grace period has elapsed since the current
* time. Once the grace-period sequence number has reached this value, it will
* be safe to invoke all callbacks that have been registered prior to the
* current time. This value is the current grace-period number plus two to the
* power of the number of low-order bits reserved for state, then rounded up to
* the next value in which the state bits are all zero.
*/
static inline unsigned long rcu_seq_snap(unsigned long *sp)
{
unsigned long s;