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Subject[PATCH 4.14 068/157] IB/hfi1: Optimize kthread pointer locking when queuing CQ entries
Date
4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Sebastian Sanchez <sebastian.sanchez@intel.com>

commit af8aab71370a692eaf7e7969ba5b1a455ac20113 upstream.

All threads queuing CQ entries on different CQs are unnecessarily
synchronized by a spin lock to check if the CQ kthread worker hasn't
been destroyed before queuing an CQ entry.

The lock used in 6efaf10f163d ("IB/rdmavt: Avoid queuing work into a
destroyed cq kthread worker") is a device global lock and will have
poor performance at scale as completions are entered from a large
number of CPUs.

Convert to use RCU where the read side of RCU is rvt_cq_enter() to
determine that the worker is alive prior to triggering the
completion event.
Apply write side RCU semantics in rvt_driver_cq_init() and
rvt_cq_exit().

Fixes: 6efaf10f163d ("IB/rdmavt: Avoid queuing work into a destroyed cq kthread worker")
Cc: <stable@vger.kernel.org> # 4.14.x
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Sebastian Sanchez <sebastian.sanchez@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/infiniband/sw/rdmavt/cq.c | 31 +++++++++++++++++++------------
include/rdma/rdma_vt.h | 2 +-
2 files changed, 20 insertions(+), 13 deletions(-)

--- a/drivers/infiniband/sw/rdmavt/cq.c
+++ b/drivers/infiniband/sw/rdmavt/cq.c
@@ -121,17 +121,20 @@ void rvt_cq_enter(struct rvt_cq *cq, str
if (cq->notify == IB_CQ_NEXT_COMP ||
(cq->notify == IB_CQ_SOLICITED &&
(solicited || entry->status != IB_WC_SUCCESS))) {
+ struct kthread_worker *worker;
+
/*
* This will cause send_complete() to be called in
* another thread.
*/
- spin_lock(&cq->rdi->n_cqs_lock);
- if (likely(cq->rdi->worker)) {
+ rcu_read_lock();
+ worker = rcu_dereference(cq->rdi->worker);
+ if (likely(worker)) {
cq->notify = RVT_CQ_NONE;
cq->triggered++;
- kthread_queue_work(cq->rdi->worker, &cq->comptask);
+ kthread_queue_work(worker, &cq->comptask);
}
- spin_unlock(&cq->rdi->n_cqs_lock);
+ rcu_read_unlock();
}

spin_unlock_irqrestore(&cq->lock, flags);
@@ -513,7 +516,7 @@ int rvt_driver_cq_init(struct rvt_dev_in
int cpu;
struct kthread_worker *worker;

- if (rdi->worker)
+ if (rcu_access_pointer(rdi->worker))
return 0;

spin_lock_init(&rdi->n_cqs_lock);
@@ -525,7 +528,7 @@ int rvt_driver_cq_init(struct rvt_dev_in
return PTR_ERR(worker);

set_user_nice(worker->task, MIN_NICE);
- rdi->worker = worker;
+ RCU_INIT_POINTER(rdi->worker, worker);
return 0;
}

@@ -537,15 +540,19 @@ void rvt_cq_exit(struct rvt_dev_info *rd
{
struct kthread_worker *worker;

- /* block future queuing from send_complete() */
- spin_lock_irq(&rdi->n_cqs_lock);
- worker = rdi->worker;
+ if (!rcu_access_pointer(rdi->worker))
+ return;
+
+ spin_lock(&rdi->n_cqs_lock);
+ worker = rcu_dereference_protected(rdi->worker,
+ lockdep_is_held(&rdi->n_cqs_lock));
if (!worker) {
- spin_unlock_irq(&rdi->n_cqs_lock);
+ spin_unlock(&rdi->n_cqs_lock);
return;
}
- rdi->worker = NULL;
- spin_unlock_irq(&rdi->n_cqs_lock);
+ RCU_INIT_POINTER(rdi->worker, NULL);
+ spin_unlock(&rdi->n_cqs_lock);
+ synchronize_rcu();

kthread_destroy_worker(worker);
}
--- a/include/rdma/rdma_vt.h
+++ b/include/rdma/rdma_vt.h
@@ -409,7 +409,7 @@ struct rvt_dev_info {
spinlock_t pending_lock; /* protect pending mmap list */

/* CQ */
- struct kthread_worker *worker; /* per device cq worker */
+ struct kthread_worker __rcu *worker; /* per device cq worker */
u32 n_cqs_allocated; /* number of CQs allocated for device */
spinlock_t n_cqs_lock; /* protect count of in use cqs */


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