[PATCH] interconnect: Don't access req_list while it's being manipulated

[Mike Tipton]

The icc_lock mutex was split into separate icc_lock and icc_bw_lock
mutexes in [1] to avoid lockdep splats. However, this didn't adequately
protect access to icc_node::req_list.

The icc_set_bw() function will eventually iterate over req_list while
only holding icc_bw_lock, but req_list can be modified while only
holding icc_lock. This causes races between icc_set_bw(), of_icc_get(),
and icc_put().

Example A:

  CPU0                               CPU1
  ----                               ----
  icc_set_bw(path_a)
    mutex_lock(&icc_bw_lock);
                                     icc_put(path_b)
                                       mutex_lock(&icc_lock);
    aggregate_requests()
      hlist_for_each_entry(r, ...
                                       hlist_del(...
        <r = invalid pointer>

Example B:

  CPU0                               CPU1
  ----                               ----
  icc_set_bw(path_a)
    mutex_lock(&icc_bw_lock);
                                     path_b = of_icc_get()
                                       of_icc_get_by_index()
                                         mutex_lock(&icc_lock);
                                         path_find()
                                           path_init()
    aggregate_requests()
      hlist_for_each_entry(r, ...
                                             hlist_add_head(...
        <r = invalid pointer>

Fix this by ensuring icc_bw_lock is always held before manipulating
icc_node::req_list. The additional places icc_bw_lock is held don't
perform any memory allocations, so we should still be safe from the
original lockdep splats that motivated the separate locks.

[1] commit af42269c3523 ("interconnect: Fix locking for runpm vs reclaim")

Signed-off-by: Mike Tipton <quic_mdtipton@xxxxxxxxxxx>
Fixes: af42269c3523 ("interconnect: Fix locking for runpm vs reclaim")
---
 drivers/interconnect/core.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/interconnect/core.c b/drivers/interconnect/core.c
index 5d1010cafed8..7e9b996b47c8 100644
--- a/drivers/interconnect/core.c
+++ b/drivers/interconnect/core.c
@@ -176,6 +176,8 @@ static struct icc_path *path_init(struct device *dev, struct icc_node *dst,
 
 	path->num_nodes = num_nodes;
 
+	mutex_lock(&icc_bw_lock);
+
 	for (i = num_nodes - 1; i >= 0; i--) {
 		node->provider->users++;
 		hlist_add_head(&path->reqs[i].req_node, &node->req_list);
@@ -186,6 +188,8 @@ static struct icc_path *path_init(struct device *dev, struct icc_node *dst,
 		node = node->reverse;
 	}
 
+	mutex_unlock(&icc_bw_lock);
+
 	return path;
 }
 
@@ -792,12 +796,16 @@ void icc_put(struct icc_path *path)
 		pr_err("%s: error (%d)\n", __func__, ret);
 
 	mutex_lock(&icc_lock);
+	mutex_lock(&icc_bw_lock);
+
 	for (i = 0; i < path->num_nodes; i++) {
 		node = path->reqs[i].node;
 		hlist_del(&path->reqs[i].req_node);
 		if (!WARN_ON(!node->provider->users))
 			node->provider->users--;
 	}
+
+	mutex_unlock(&icc_bw_lock);
 	mutex_unlock(&icc_lock);
 
 	kfree_const(path->name);
-- 
2.17.1