[PATCH 5/5] srcu: Explain why callbacks invocations can't run concurrently

Frederic Weisbecker
2023-10-03T23:29:37Z

If an SRCU barrier is queued while callbacks are running and a new
callbacks invocator for the same sdp were to run concurrently, the
RCU barrier might execute too early. As this requirement is non-obvious,
make sure to keep a record.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
kernel/rcu/srcutree.c | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 2bfc8ed1eed2..0351a4e83529 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -1715,6 +1715,11 @@ static void srcu_invoke_callbacks(struct work_struct *work)
WARN_ON_ONCE(!rcu_segcblist_segempty(&sdp->srcu_cblist, RCU_NEXT_TAIL));
rcu_segcblist_advance(&sdp->srcu_cblist,
rcu_seq_current(&ssp->srcu_sup->srcu_gp_seq));
+ /*
+ * Although this function is theoretically re-entrant, concurrent
+ * callbacks invocation is disallowed to avoid executing an SRCU barrier
+ * too early.
+ */
if (sdp->srcu_cblist_invoking ||
!rcu_segcblist_ready_cbs(&sdp->srcu_cblist)) {
spin_unlock_irq_rcu_node(sdp);
@@ -1745,6 +1750,7 @@ static void srcu_invoke_callbacks(struct work_struct *work)
sdp->srcu_cblist_invoking = false;
more = rcu_segcblist_ready_cbs(&sdp->srcu_cblist);
spin_unlock_irq_rcu_node(sdp);
+ /* An SRCU barrier or callbacks from previous nesting work pending */
if (more)
srcu_schedule_cbs_sdp(sdp, 0);
}
--
2.41.0