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Subject[RFC v5 3/5] rcu: Fix rcu_barrier() breakage from earlier patch
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This patch is split up because I'm not sure about the patch, and also because
it does not fix the issue I am seeing with TREE04. :-(. Though it does fix the
theoretical issue I was considering, with rcu_barrier.

The previous patch breaks rcu_barrier (in theory at least). This is because
rcu_barrier() may skip queuing a callback and return too early. Fix it by storing
state to indicate that callbacks are being invoked and the callback list should
not appear as non-empty. This is a terrible hack, however it still does not fix
TREE04.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
include/linux/rcu_segcblist.h | 1 +
kernel/rcu/rcu_segcblist.h | 23 +++++++++++++++++++++--
kernel/rcu/tree.c | 4 ++++
3 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/include/linux/rcu_segcblist.h b/include/linux/rcu_segcblist.h
index d462ae5e340a..319a565f6ecb 100644
--- a/include/linux/rcu_segcblist.h
+++ b/include/linux/rcu_segcblist.h
@@ -76,6 +76,7 @@ struct rcu_segcblist {
#endif
u8 enabled;
u8 offloaded;
+ u8 invoking;
};

#define RCU_SEGCBLIST_INITIALIZER(n) \
diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h
index 7f4e02bbb806..78949e125364 100644
--- a/kernel/rcu/rcu_segcblist.h
+++ b/kernel/rcu/rcu_segcblist.h
@@ -40,14 +40,33 @@ static inline bool rcu_segcblist_empty(struct rcu_segcblist *rsclp)
return !READ_ONCE(rsclp->head);
}

+static inline void rcu_segcblist_set_invoking(struct rcu_segcblist *rsclp)
+{
+ WRITE_ONCE(rsclp->invoking, 1);
+}
+
+static inline void rcu_segcblist_reset_invoking(struct rcu_segcblist *rsclp)
+{
+ WRITE_ONCE(rsclp->invoking, 0);
+}
+
/* Return number of callbacks in segmented callback list. */
static inline long rcu_segcblist_n_cbs(struct rcu_segcblist *rsclp)
{
+ long ret;
#ifdef CONFIG_RCU_NOCB_CPU
- return atomic_long_read(&rsclp->len);
+ ret = atomic_long_read(&rsclp->len);
#else
- return READ_ONCE(rsclp->len);
+ ret = READ_ONCE(rsclp->len);
#endif
+
+ /*
+ * An invoking list should not appear empty. This is required
+ * by rcu_barrier().
+ */
+ if (ret)
+ return ret;
+ return READ_ONCE(rsclp->invoking) ? 1 : 0;
}

/*
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index ab4d4e9ff549..23fb6d7b6d4a 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2461,6 +2461,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
}
trace_rcu_batch_start(rcu_state.name,
rcu_segcblist_n_cbs(&rdp->cblist), bl);
+ rcu_segcblist_set_invoking(&rdp->cblist);
rcu_segcblist_extract_done_cbs(&rdp->cblist, &rcl);
if (offloaded)
rdp->qlen_last_fqs_check = rcu_segcblist_n_cbs(&rdp->cblist);
@@ -2517,6 +2518,9 @@ static void rcu_do_batch(struct rcu_data *rdp)
/* Update counts and requeue any remaining callbacks. */
rcu_segcblist_insert_done_cbs(&rdp->cblist, &rcl);

+ smp_mb();
+ rcu_segcblist_reset_invoking(&rdp->cblist);
+
/* Reinstate batch limit if we have worked down the excess. */
count = rcu_segcblist_n_cbs(&rdp->cblist);
if (rdp->blimit >= DEFAULT_MAX_RCU_BLIMIT && count <= qlowmark)
--
2.28.0.681.g6f77f65b4e-goog
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