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SubjectRe: [RFC PATCH 1/5] kernel/rcu/tree.c:1272 fix a sparse warning
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
>
> We can simplify the function by keeping the contexts together and removing
> redundant checks.
>
> Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
> ---
> kernel/rcu/tree.c | 65 ++++++++++++++++++++++++++++++-------------------------
> 1 file changed, 35 insertions(+), 30 deletions(-)
>
> 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.

> * 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?

> + /*
> + * 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
> + */
> + c = rcu_cbs_completed(rdp->rsp, rnp_root);

But I believe that this statement could be moved into the preceding "if"
statement in the original code. If this is really the case, it could
be a good change.

Thanx, Paul

> +
> + /*
> + * If the needed request 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"));
> + raw_spin_unlock(&rnp_root->lock);
> + goto out;
> + }
> +
> + /* Record the need for the future grace period. */
> + rnp_root->need_future_gp[c & 0x1]++;
> +
> + /*
> + * Start a new grace period since it is not started
> + */
> + trace_rcu_future_gp(rnp, rdp, c, TPS("Startedroot"));
> + ret = rcu_start_gp_advanced(rdp->rsp, rnp_root, rdp);
> + raw_spin_unlock(&rnp_root->lock);
> + goto out;
> }
>
> + /* rnp == rnp_root, we already hold the lock */
> + trace_rcu_future_gp(rnp, rdp, c, TPS("StartedLeaf"));
> + ret = rcu_start_gp_advanced(rdp->rsp, rnp, rdp);
> +out:
> /*
> - * 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.
> + * 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;
> - }
> -
> - /* 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);
> -out:
> if (c_out != NULL)
> *c_out = c;
> return ret;
> --
> 1.9.1
>



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