diff options
Diffstat (limited to 'kernel/rcu')
| -rw-r--r-- | kernel/rcu/srcutree.c | 24 |
1 files changed, 20 insertions, 4 deletions
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index 0351a4e83529..e4d673fc30f4 100644 --- a/kernel/rcu/srcutree.c +++ b/kernel/rcu/srcutree.c @@ -1234,11 +1234,20 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp, if (rhp) rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp); /* - * The snapshot for acceleration must be taken _before_ the read of the - * current gp sequence used for advancing, otherwise advancing may fail - * and acceleration may then fail too. + * It's crucial to capture the snapshot 's' for acceleration before + * reading the current gp_seq that is used for advancing. This is + * essential because if the acceleration snapshot is taken after a + * failed advancement attempt, there's a risk that a grace period may + * conclude and a new one may start in the interim. If the snapshot is + * captured after this sequence of events, the acceleration snapshot 's' + * could be excessively advanced, leading to acceleration failure. + * In such a scenario, an 'acceleration leak' can occur, where new + * callbacks become indefinitely stuck in the RCU_NEXT_TAIL segment. + * Also note that encountering advancing failures is a normal + * occurrence when the grace period for RCU_WAIT_TAIL is in progress. * - * This could happen if: + * To see this, consider the following events which occur if + * rcu_seq_snap() were to be called after advance: * * 1) The RCU_WAIT_TAIL segment has callbacks (gp_num = X + 4) and the * RCU_NEXT_READY_TAIL also has callbacks (gp_num = X + 8). @@ -1264,6 +1273,13 @@ static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp, if (rhp) { rcu_segcblist_advance(&sdp->srcu_cblist, rcu_seq_current(&ssp->srcu_sup->srcu_gp_seq)); + /* + * Acceleration can never fail because the base current gp_seq + * used for acceleration is <= the value of gp_seq used for + * advancing. This means that RCU_NEXT_TAIL segment will + * always be able to be emptied by the acceleration into the + * RCU_NEXT_READY_TAIL or RCU_WAIT_TAIL segments. + */ WARN_ON_ONCE(!rcu_segcblist_accelerate(&sdp->srcu_cblist, s)); } if (ULONG_CMP_LT(sdp->srcu_gp_seq_needed, s)) { |