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Message-ID: <539A83F3.2060407@gmail.com>
Date: Fri, 13 Jun 2014 00:54:11 -0400
From: Pranith Kumar <bobby.prani@...il.com>
To: paulmck@...ux.vnet.ibm.com
CC: linux-kernel@...r.kernel.org, Josh Triplett <josh@...htriplett.org>
Subject: Re: [RFC PATCH 1/5] kernel/rcu/tree.c:1272 fix a sparse warning
On 06/12/2014 07:16 PM, Paul E. McKenney wrote:
> On Wed, Jun 11, 2014 at 04:39:39PM -0400, Pranith Kumar wrote:
>> kernel/rcu/tree.c:1272:9: warning: context imbalance in 'rcu_start_future_gp' - different lock contexts for basic block
>>
>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>> index f1ba773..9ab84d3 100644
>> --- a/kernel/rcu/tree.c
>> +++ b/kernel/rcu/tree.c
>> @@ -1234,49 +1234,54 @@ rcu_start_future_gp(struct rcu_node *rnp, struct rcu_data *rdp,
>> }
>>
>> /*
>> - * There might be no grace period in progress. If we don't already
>> + * There is be no grace period in progress. If we don't already
>
> We actually don't know at this point, unless rnp==rnp_root. Otherwise,
> the grace period might have started, but initialization might not yet
> have reached rnp.
I should have mentioned that I wrote this on top of the previous patch where we
were checking the root node for presence of a grace period
ACCESS_ONCE(rnp_root->gpnum) != ACCESS_ONCE(rnp_root->completed)
But, I realize that even this does not guarantee that a grace period is in
progress as we do not yet have the lock for the root.
>
>> * hold it, acquire the root rcu_node structure's lock in order to
>> - * start one (if needed).
>> + * start one.
>> */
>> if (rnp != rnp_root) {
>> raw_spin_lock(&rnp_root->lock);
>> smp_mb__after_unlock_lock();
>
> I am not convinced that this transformation is correct, especially in
> the rnp==rnp_root case. For one thing, I don't see the need for a
> future grace period being recorded in that case.
>
> And I believe that if this transformation is fixed, there will be some
> duplicate code, which scares me more than sparse false positives. So I
> am not willing to take this sort of transformation. Or am I missing
> something?
>
You are right. I knew I missed something! Even though this started as an
exercise to remove the sparse warning, I thought I could simplify the function
since I could see that we are doing some things twice.
Please find v2 below which takes care of the issues you mentioned. RFC please!
simplify the rcu_start_future_gp function. fix sparse warning as an added bonus :)
Signed-off-by: Pranith Kumar <bobby.prani@...il.com>
---
kernel/rcu/tree.c | 80 +++++++++++++++++++++++++------------------------------
1 file changed, 37 insertions(+), 43 deletions(-)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index f1ba773..ee98d0b 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1192,44 +1192,60 @@ static void trace_rcu_future_gp(struct rcu_node *rnp, struct rcu_data *rdp,
}
/*
+ * Adjust callbacks as needed. Note that even no-CBs CPUs
+ * have a ->nxtcompleted[] array, so no no-CBs checks needed.
+ */
+static void rcu_adjust_callbacks(unsigned long c, struct rcu_data *rdp)
+{
+ int i;
+ for (i = RCU_DONE_TAIL; i < RCU_NEXT_TAIL; i++)
+ if (ULONG_CMP_LT(c, rdp->nxtcompleted[i]))
+ rdp->nxtcompleted[i] = c;
+}
+
+/*
* Start some future grace period, as needed to handle newly arrived
* callbacks. The required future grace periods are recorded in each
* rcu_node structure's ->need_future_gp field. Returns true if there
* is reason to awaken the grace-period kthread.
*
* The caller must hold the specified rcu_node structure's ->lock.
+ *
+ * This is called recursively at-most twice, once with a rcu_node and
+ * once with the root rcu_node.
*/
static bool __maybe_unused
rcu_start_future_gp(struct rcu_node *rnp, struct rcu_data *rdp,
unsigned long *c_out)
{
unsigned long c;
- int i;
bool ret = false;
struct rcu_node *rnp_root = rcu_get_root(rdp->rsp);
+ bool is_root = (rnp_root == rnp);
/*
* Pick up grace-period number for new callbacks. If this
* grace period is already marked as needed, return to the caller.
*/
c = rcu_cbs_completed(rdp->rsp, rnp);
- trace_rcu_future_gp(rnp, rdp, c, TPS("Startleaf"));
+ trace_rcu_future_gp(rnp, rdp, c,
+ is_root ? TPS("Startedroot") : TPS("Startleaf"));
if (rnp->need_future_gp[c & 0x1]) {
- trace_rcu_future_gp(rnp, rdp, c, TPS("Prestartleaf"));
+ trace_rcu_future_gp(rnp, rdp, c,
+ is_root ? TPS("Prestartroot") : TPS("Prestartleaf"));
goto out;
}
/*
- * If either this rcu_node structure or the root rcu_node structure
- * believe that a grace period is in progress, then we must wait
- * for the one following, which is in "c". Because our request
- * will be noticed at the end of the current grace period, we don't
- * need to explicitly start one.
+ * If this rcu_node structure believes that a grace period is in progress,
+ * then we must wait for the one following, which is in "c".
+ * Because our request will be noticed at the end of the current grace
+ * period, we don't need to explicitly start one.
*/
- if (rnp->gpnum != rnp->completed ||
- ACCESS_ONCE(rnp->gpnum) != ACCESS_ONCE(rnp->completed)) {
+ if (rnp->gpnum != rnp->completed) {
rnp->need_future_gp[c & 0x1]++;
- trace_rcu_future_gp(rnp, rdp, c, TPS("Startedleaf"));
+ trace_rcu_future_gp(rnp, rdp, c,
+ is_root ? TPS("Startedleafroot") : TPS("Startleaf"));
goto out;
}
@@ -1241,41 +1257,19 @@ rcu_start_future_gp(struct rcu_node *rnp, struct rcu_data *rdp,
if (rnp != rnp_root) {
raw_spin_lock(&rnp_root->lock);
smp_mb__after_unlock_lock();
- }
-
- /*
- * Get a new grace-period number. If there really is no grace
- * period in progress, it will be smaller than the one we obtained
- * earlier. Adjust callbacks as needed. Note that even no-CBs
- * CPUs have a ->nxtcompleted[] array, so no no-CBs checks needed.
- */
- c = rcu_cbs_completed(rdp->rsp, rnp_root);
- for (i = RCU_DONE_TAIL; i < RCU_NEXT_TAIL; i++)
- if (ULONG_CMP_LT(c, rdp->nxtcompleted[i]))
- rdp->nxtcompleted[i] = c;
- /*
- * If the needed for the required grace period is already
- * recorded, trace and leave.
- */
- if (rnp_root->need_future_gp[c & 0x1]) {
- trace_rcu_future_gp(rnp, rdp, c, TPS("Prestartedroot"));
- goto unlock_out;
+ /*
+ * Start a new grace period using the root node
+ */
+ ret = rcu_start_future_gp(rnp_root, rdp, &c);
+ raw_spin_unlock(&rnp_root->lock);
+ goto out;
}
- /* Record the need for the future grace period. */
- rnp_root->need_future_gp[c & 0x1]++;
-
- /* If a grace period is not already in progress, start one. */
- if (rnp_root->gpnum != rnp_root->completed) {
- trace_rcu_future_gp(rnp, rdp, c, TPS("Startedleafroot"));
- } else {
- trace_rcu_future_gp(rnp, rdp, c, TPS("Startedroot"));
- ret = rcu_start_gp_advanced(rdp->rsp, rnp_root, rdp);
- }
-unlock_out:
- if (rnp != rnp_root)
- raw_spin_unlock(&rnp_root->lock);
+ rcu_adjust_callbacks(c, rdp);
+ /* rnp == rnp_root, we already hold the lock */
+ trace_rcu_future_gp(rnp, rdp, c, TPS("Startedroot"));
+ ret = rcu_start_gp_advanced(rdp->rsp, rnp, rdp);
out:
if (c_out != NULL)
*c_out = c;
--
1.9.1
--
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