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Subject[PATCH tip/core/rcu 14/14] rcu: Make rcu_accelerate_cbs() note need for future grace periods
Date
From: "Paul E. McKenney" <paul.mckenney@linaro.org>

Now that rcu_start_future_gp() has been abstracted from
rcu_nocb_wait_gp(), rcu_accelerate_cbs() can invoke rcu_start_future_gp()
so as to register the need for any future grace periods needed by a
CPU about to enter dyntick-idle mode. This commit makes this change.
Note that some refactoring of rcu_start_gp() is carried out to avoid
recursion and subsequent self-deadlocks.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
kernel/rcutree.c | 50 ++++++++++++++++++++++++++++++++------------------
1 files changed, 32 insertions(+), 18 deletions(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index bd42feb..f7399b4 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -230,7 +230,8 @@ static ulong jiffies_till_next_fqs = RCU_JIFFIES_TILL_FORCE_QS;
module_param(jiffies_till_first_fqs, ulong, 0644);
module_param(jiffies_till_next_fqs, ulong, 0644);

-static void rcu_start_gp(struct rcu_state *rsp);
+static void rcu_start_gp_advanced(struct rcu_state *rsp, struct rcu_node *rnp,
+ struct rcu_data *rdp);
static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *));
static void force_quiescent_state(struct rcu_state *rsp);
static int rcu_pending(int cpu);
@@ -1200,7 +1201,7 @@ rcu_start_future_gp(struct rcu_node *rnp, struct rcu_data *rdp)
trace_rcu_future_gp(rnp, rdp, c, "Startedleafroot");
} else {
trace_rcu_future_gp(rnp, rdp, c, "Startedroot");
- rcu_start_gp(rdp->rsp);
+ rcu_start_gp_advanced(rdp->rsp, rnp_root, rdp);
}
unlock_out:
if (rnp != rnp_root)
@@ -1286,6 +1287,8 @@ static void rcu_accelerate_cbs(struct rcu_state *rsp, struct rcu_node *rnp,
rdp->nxttail[i] = rdp->nxttail[RCU_NEXT_TAIL];
rdp->nxtcompleted[i] = c;
}
+ /* Record any needed additional grace periods. */
+ rcu_start_future_gp(rnp, rdp);

/* Trace depending on how much we were able to accelerate. */
if (!*rdp->nxttail[RCU_WAIT_TAIL])
@@ -1647,20 +1650,9 @@ static int __noreturn rcu_gp_kthread(void *arg)
* quiescent state.
*/
static void
-rcu_start_gp(struct rcu_state *rsp)
+rcu_start_gp_advanced(struct rcu_state *rsp, struct rcu_node *rnp,
+ struct rcu_data *rdp)
{
- struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
- struct rcu_node *rnp = rcu_get_root(rsp);
-
- /*
- * If there is no grace period in progress right now, any
- * callbacks we have up to this point will be satisfied by the
- * next grace period. Also, advancing the callbacks reduces the
- * probability of false positives from cpu_needs_another_gp()
- * resulting in pointless grace periods. So, advance callbacks!
- */
- rcu_advance_cbs(rsp, rnp, rdp);
-
if (!rsp->gp_kthread || !cpu_needs_another_gp(rsp, rdp)) {
/*
* Either we have not yet spawned the grace-period
@@ -1672,14 +1664,36 @@ rcu_start_gp(struct rcu_state *rsp)
}
rsp->gp_flags = RCU_GP_FLAG_INIT;

- /* Ensure that CPU is aware of completion of last grace period. */
- __rcu_process_gp_end(rsp, rdp->mynode, rdp);
-
/* Wake up rcu_gp_kthread() to start the grace period. */
wake_up(&rsp->gp_wq);
}

/*
+ * Similar to rcu_start_gp_advanced(), but also advance the calling CPU's
+ * callbacks. Note that rcu_start_gp_advanced() cannot do this because it
+ * is invoked indirectly from rcu_advance_cbs(), which would result in
+ * endless recursion -- or would do so if it wasn't for the self-deadlock
+ * that is encountered beforehand.
+ */
+static void
+rcu_start_gp(struct rcu_state *rsp)
+{
+ struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
+ struct rcu_node *rnp = rcu_get_root(rsp);
+
+ /*
+ * If there is no grace period in progress right now, any
+ * callbacks we have up to this point will be satisfied by the
+ * next grace period. Also, advancing the callbacks reduces the
+ * probability of false positives from cpu_needs_another_gp()
+ * resulting in pointless grace periods. So, advance callbacks
+ * then start the grace period!
+ */
+ rcu_advance_cbs(rsp, rnp, rdp);
+ rcu_start_gp_advanced(rsp, rnp, rdp);
+}
+
+/*
* Report a full set of quiescent states to the specified rcu_state
* data structure. This involves cleaning up after the prior grace
* period and letting rcu_start_gp() start up the next grace period
--
1.7.8


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