gpu: nvgpu: remove rcu locks in clk arbiter

RCU's are available only in (linux) kernel. Though they are
able to achieve lockless access in some specific scenarios,
they are heavily dependent on the kernel for their functionality.
E.g. synchronize_rcu(), which depends on the kernel in order to
delimit read side critical sections.

As such it is very difficult to implement constructs analogous
to RCUs in userspace code. As a result the code which depends on
RCU's for synchronization is not portable between OS'es,
especially if one of them is in userspace, viz. QNX.

Also, if the code is not in performance critical path, we can do
with non-RCU constructs.

For clk arbiter code here, RCU's are replaced by the traditional
spinlocks, so that the code could be used by QNX down the line.

Jira VQRM-3741

Change-Id: I178e5958788c8fd998303a6a94d8f2f328201508
Signed-off-by: Sourab Gupta <sourabg@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/1705535
Reviewed-by: svc-mobile-coverity <svc-mobile-coverity@nvidia.com>
Reviewed-by: Alex Waterman <alexw@nvidia.com>
GVS: Gerrit_Virtual_Submit
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

1 files changed, 34 insertions, 33 deletions

diff --git a/drivers/gpu/nvgpu/common/linux/clk_arb.c b/drivers/gpu/nvgpu/common/linux/clk_arb.c
index 39763f14..ec40a6ce 100644
--- a/drivers/gpu/nvgpu/common/linux/clk_arb.c
+++ b/drivers/gpu/nvgpu/common/linux/clk_arb.c
@@ -16,9 +16,8 @@
 
 #include <linux/cdev.h>
 #include <linux/file.h>
+#include <linux/list.h>
 #include <linux/anon_inodes.h>
-#include <linux/rculist.h>
-#include <linux/llist.h>
 #include <linux/uaccess.h>
 
 #include <nvgpu/bitops.h>
@@ -740,7 +739,6 @@ static void nvgpu_clk_arb_run_arbiter_cb(struct work_struct *work)
         struct nvgpu_clk_dev *tmp;
         struct nvgpu_clk_arb_target *target, *actual;
         struct gk20a *g = arb->g;
-        struct llist_node *head;
 
         u32 pstate = VF_POINT_INVALID_PSTATE;
         u32 voltuv, voltuv_sram;
@@ -775,25 +773,21 @@ static void nvgpu_clk_arb_run_arbiter_cb(struct work_struct *work)
         gpc2clk_target = 0;
         mclk_target = 0;
 
-        rcu_read_lock();
-        list_for_each_entry_rcu(session, &arb->sessions, link) {
+        nvgpu_spinlock_acquire(&arb->sessions_lock);
+        list_for_each_entry(session, &arb->sessions, link) {
                 if (!session->zombie) {
                         mclk_set = false;
                         gpc2clk_set = false;
-                        target = NV_ACCESS_ONCE(session->target) ==
-                                &session->target_pool[0] ?
+                        target = (session->target == &session->target_pool[0] ?
                                         &session->target_pool[1] :
-                                        &session->target_pool[0];
-                        /* Do not reorder pointer */
-                        nvgpu_smp_rmb();
-                        head = llist_del_all(&session->targets);
-                        if (head) {
-
+                                        &session->target_pool[0]);
+                        nvgpu_spinlock_acquire(&session->session_lock);
+                        if (!list_empty(&session->targets)) {
                                 /* Copy over state */
                                 target->mclk = session->target->mclk;
                                 target->gpc2clk = session->target->gpc2clk;
                                 /* Query the latest committed request */
-                                llist_for_each_entry_safe(dev, tmp, head,
+                                list_for_each_entry_safe(dev, tmp, &session->targets,
                                                                         node) {
                                         if (!mclk_set && dev->mclk_target_mhz) {
                                                 target->mclk =
@@ -807,12 +801,14 @@ static void nvgpu_clk_arb_run_arbiter_cb(struct work_struct *work)
                                                 gpc2clk_set = true;
                                         }
                                         nvgpu_ref_get(&dev->refcount);
-                                        llist_add(&dev->node, &arb->requests);
+                                        list_del(&dev->node);
+                                        nvgpu_spinlock_acquire(&arb->requests_lock);
+                                        list_add(&dev->node, &arb->requests);
+                                        nvgpu_spinlock_release(&arb->requests_lock);
                                 }
-                                /* Ensure target is updated before ptr sawp */
-                                nvgpu_smp_wmb();
                                 xchg(&session->target, target);
                         }
+                        nvgpu_spinlock_release(&session->session_lock);
 
                         mclk_target = mclk_target > session->target->mclk ?
                                 mclk_target : session->target->mclk;
@@ -822,7 +818,7 @@ static void nvgpu_clk_arb_run_arbiter_cb(struct work_struct *work)
                                 gpc2clk_target : session->target->gpc2clk;
                 }
         }
-        rcu_read_unlock();
+        nvgpu_spinlock_release(&arb->sessions_lock);
 
         gpc2clk_target = (gpc2clk_target > 0) ? gpc2clk_target :
                         arb->gpc2clk_default_mhz;
@@ -1010,22 +1006,24 @@ exit_arb:
 
         current_alarm = (u32) nvgpu_atomic64_read(&arb->alarm_mask);
         /* notify completion for all requests */
-        head = llist_del_all(&arb->requests);
-        llist_for_each_entry_safe(dev, tmp, head, node) {
+        nvgpu_spinlock_acquire(&arb->requests_lock);
+        list_for_each_entry_safe(dev, tmp, &arb->requests, node) {
                 nvgpu_atomic_set(&dev->poll_mask, NVGPU_POLLIN | NVGPU_POLLRDNORM);
                 nvgpu_cond_signal_interruptible(&dev->readout_wq);
                 nvgpu_ref_put(&dev->refcount, nvgpu_clk_arb_free_fd);
+                list_del(&dev->node);
         }
+        nvgpu_spinlock_release(&arb->requests_lock);
 
         nvgpu_atomic_set(&arb->notification_queue.head,
                 nvgpu_atomic_read(&arb->notification_queue.tail));
         /* notify event for all users */
-        rcu_read_lock();
-        list_for_each_entry_rcu(dev, &arb->users, link) {
+        nvgpu_spinlock_acquire(&arb->users_lock);
+        list_for_each_entry(dev, &arb->users, link) {
                 alarms_notified |=
                         nvgpu_clk_arb_notify(dev, arb->actual, current_alarm);
         }
-        rcu_read_unlock();
+        nvgpu_spinlock_release(&arb->users_lock);
 
         /* clear alarms */
         nvgpu_clk_arb_clear_global_alarm(g, alarms_notified &
@@ -1054,6 +1052,7 @@ int nvgpu_clk_arb_init_arbiter(struct gk20a *g)
                 goto mutex_fail;
         nvgpu_spinlock_init(&arb->sessions_lock);
         nvgpu_spinlock_init(&arb->users_lock);
+        nvgpu_spinlock_init(&arb->requests_lock);
 
         arb->mclk_f_points = nvgpu_kcalloc(g, MAX_F_POINTS, sizeof(u16));
         if (!arb->mclk_f_points) {
@@ -1119,9 +1118,9 @@ int nvgpu_clk_arb_init_arbiter(struct gk20a *g)
         if (err < 0)
                 goto init_fail;
 
-        INIT_LIST_HEAD_RCU(&arb->users);
-        INIT_LIST_HEAD_RCU(&arb->sessions);
-        init_llist_head(&arb->requests);
+        INIT_LIST_HEAD(&arb->users);
+        INIT_LIST_HEAD(&arb->sessions);
+        INIT_LIST_HEAD(&arb->requests);
 
         nvgpu_cond_init(&arb->request_wq);
         arb->vf_table_work_queue = alloc_workqueue("%s", WQ_HIGHPRI, 1,
@@ -1245,10 +1244,11 @@ int nvgpu_clk_arb_init_session(struct gk20a *g,
         nvgpu_smp_wmb();
         session->target = &session->target_pool[0];
 
-        init_llist_head(&session->targets);
+        INIT_LIST_HEAD(&session->targets);
+        nvgpu_spinlock_init(&session->session_lock);
 
         nvgpu_spinlock_acquire(&arb->sessions_lock);
-        list_add_tail_rcu(&session->link, &arb->sessions);
+        list_add_tail(&session->link, &arb->sessions);
         nvgpu_spinlock_release(&arb->sessions_lock);
 
         *_session = session;
@@ -1272,21 +1272,22 @@ void nvgpu_clk_arb_free_session(struct nvgpu_ref *refcount)
         struct nvgpu_clk_arb *arb = session->g->clk_arb;
         struct gk20a *g = session->g;
         struct nvgpu_clk_dev *dev, *tmp;
-        struct llist_node *head;
 
         nvgpu_log(g, gpu_dbg_fn | gpu_dbg_clk_arb, " ");
 
         if (arb) {
                 nvgpu_spinlock_acquire(&arb->sessions_lock);
-                list_del_rcu(&session->link);
+                list_del(&session->link);
                 nvgpu_spinlock_release(&arb->sessions_lock);
         }
 
-        head = llist_del_all(&session->targets);
-        llist_for_each_entry_safe(dev, tmp, head, node) {
+        nvgpu_spinlock_acquire(&session->session_lock);
+        list_for_each_entry_safe(dev, tmp, &session->targets, node) {
                 nvgpu_ref_put(&dev->refcount, nvgpu_clk_arb_free_fd);
+                list_del(&dev->node);
         }
-        synchronize_rcu();
+        nvgpu_spinlock_release(&session->session_lock);
+
         nvgpu_kfree(g, session);
 }