[PATCH] rcu: Start grace period even for single callback

[Kishan Kumar <kishank@codeaurora.org>]

When we queue a callback for RCU, it goes and sits
on the nxttail of the per-cpu RCU data structure.
Callback is represented via struct callback_head
structure.

struct callback_head {
        struct callback_head *next;
        void (*func)(struct callback_head *head);
};

In case of a single callback queued in the nxttail,
the next field will be NULL. "next" happens to be
the zeroeth element of struct callback_head.

The condition "if(*rdp->nxttail[RCU_NEXT_READY_TAIL])"
in the function cpu_needs_another_gp(), essentially
checks if any callback is queued.

Since *rdp->nxttail[RCU_NEXT_READY_TAIL] dereferences
to the first element, the if condition will just turn
out to be if(NULL) in case there is a single callback
queued. This in turn causes cpu_needs_another_gp() to
return false even though we need a grace period to
process the single callback. This leads to writers
waiting until a second call_back gets queued, which can
cause undesirable effects like boot up delay upto 300
seconds, etc.

Fix this by performing this check on the "func" field
rather than the "next" field of the callback_head.

Signed-off-by: Kishan Kumar <kishank@codeaurora.org>
Signed-off-by: Mohammed Khajapasha <mkhaja@codeaurora.org>
Signed-off-by: Vignesh Radhakrishnan <vigneshr@codeaurora.org>
---
 kernel/rcu/tree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index fc3abf1..394a4fa 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -324,7 +324,7 @@ cpu_needs_another_gp(struct rcu_state *rsp, struct rcu_data *rdp)
 		return 1;  /* Yes, a no-CBs CPU needs one. */
 	if (!rdp->nxttail[RCU_NEXT_TAIL])
 		return 0;  /* No, this is a no-CBs (or offline) CPU. */
-	if (*rdp->nxttail[RCU_NEXT_READY_TAIL])
+	if (((struct callback_head *)rdp->nxttail[RCU_NEXT_READY_TAIL])->func)
 		return 1;  /* Yes, this CPU has newly registered callbacks. */
 	for (i = RCU_WAIT_TAIL; i < RCU_NEXT_TAIL; i++)
 		if (rdp->nxttail[i - 1] != rdp->nxttail[i] &&
-- 
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