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rcu: Make nocb_gp_wait() double-check unexpected-callback warning

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Currently, nocb_gp_wait() unconditionally complains if there is a
callback not already associated with a grace period.  This assumes that
either there was no such callback initially on the one hand, or that
the rcu_advance_cbs() function assigned all such callbacks to a grace
period on the other.  However, in theory there are some situations that
would prevent rcu_advance_cbs() from assigning all of the callbacks.

This commit therefore checks for unassociated callbacks immediately after
rcu_advance_cbs() returns, while the corresponding rcu_node structure's
->lock is still held.  If there are unassociated callbacks at that point,
the subsequent WARN_ON_ONCE() is disabled.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Paul E. McKenney 2020-02-04 14:55:29 -08:00
parent 13817dd589
commit 3d05031ae6
1 changed files with 5 additions and 1 deletions
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6
kernel/rcu/tree_plugin.h
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@ -1931,6 +1931,7 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
struct rcu_data *rdp;
struct rcu_node *rnp;
unsigned long wait_gp_seq = 0; // Suppress "use uninitialized" warning.
bool wasempty = false;
/*
* Each pass through the following loop checks for CBs and for the
@ -1970,10 +1971,13 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
rcu_seq_done(&rnp->gp_seq, cur_gp_seq))) {
raw_spin_lock_rcu_node(rnp); /* irqs disabled. */
needwake_gp = rcu_advance_cbs(rnp, rdp);
wasempty = rcu_segcblist_restempty(&rdp->cblist,
RCU_NEXT_READY_TAIL);
raw_spin_unlock_rcu_node(rnp); /* irqs disabled. */
}
// Need to wait on some grace period?
WARN_ON_ONCE(!rcu_segcblist_restempty(&rdp->cblist,
WARN_ON_ONCE(wasempty &&
!rcu_segcblist_restempty(&rdp->cblist,
RCU_NEXT_READY_TAIL));
if (rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq)) {
if (!needwait_gp ||