Waiting for activity on the "clean_list" to quiesce is no substitute
for proper locking.
We can have multiple threads competing for "llist_del_first"
via "rds_ib_reuse_mr", and a single thread competing
for "llist_del_all" and "llist_del_first" via "rds_ib_flush_mr_pool".
Since "llist_del_first" depends on "list->first->next" not to change
in the midst of the operation, simply waiting for all current calls
to "rds_ib_reuse_mr" to quiesce across all CPUs is woefully inadequate:
By the time "wait_clean_list_grace" is done iterating over all CPUs to see
that there is no concurrent caller to "rds_ib_reuse_mr", a new caller may
have just shown up on the first CPU.
Furthermore, <linux/llist.h> explicitly calls out the need for locking:
* Cases where locking is needed:
* If we have multiple consumers with llist_del_first used in one consumer,
* and llist_del_first or llist_del_all used in other consumers,
* then a lock is needed.
Also, while at it, drop the unused "pool" parameter
from "list_to_llist_nodes".
Signed-off-by: Gerd Rausch <gerd.rausch@xxxxxxxxxx>
---
net/rds/ib_mr.h | 1 +
net/rds/ib_rdma.c | 56 +++++++++++++++--------------------------------
2 files changed, 19 insertions(+), 38 deletions(-)
diff --git a/net/rds/ib_mr.h b/net/rds/ib_mr.h
index 42daccb7b5eb..ab26c20ed66f 100644
--- a/net/rds/ib_mr.h
+++ b/net/rds/ib_mr.h
@@ -98,6 +98,7 @@ struct rds_ib_mr_pool {
struct llist_head free_list; /* unused MRs */
struct llist_head clean_list; /* unused & unmapped MRs */
wait_queue_head_t flush_wait;
+ spinlock_t clean_lock; /* "clean_list" concurrency */
atomic_t free_pinned; /* memory pinned by free MRs */
unsigned long max_items;
diff --git a/net/rds/ib_rdma.c b/net/rds/ib_rdma.c
index 0b347f46b2f4..6b047e63a769 100644
--- a/net/rds/ib_rdma.c
+++ b/net/rds/ib_rdma.c
@@ -40,9 +40,6 @@
struct workqueue_struct *rds_ib_mr_wq;
-static DEFINE_PER_CPU(unsigned long, clean_list_grace);
-#define CLEAN_LIST_BUSY_BIT 0
-
static struct rds_ib_device *rds_ib_get_device(__be32 ipaddr)
{
struct rds_ib_device *rds_ibdev;
@@ -195,12 +192,11 @@ struct rds_ib_mr *rds_ib_reuse_mr(struct rds_ib_mr_pool *pool)
{
struct rds_ib_mr *ibmr = NULL;
struct llist_node *ret;
- unsigned long *flag;
+ unsigned long flags;
- preempt_disable();
- flag = this_cpu_ptr(&clean_list_grace);
- set_bit(CLEAN_LIST_BUSY_BIT, flag);
+ spin_lock_irqsave(&pool->clean_lock, flags);
ret = llist_del_first(&pool->clean_list);
+ spin_unlock_irqrestore(&pool->clean_lock, flags);
if (ret) {
ibmr = llist_entry(ret, struct rds_ib_mr, llnode);
if (pool->pool_type == RDS_IB_MR_8K_POOL)
@@ -209,23 +205,9 @@ struct rds_ib_mr *rds_ib_reuse_mr(struct rds_ib_mr_pool *pool)
rds_ib_stats_inc(s_ib_rdma_mr_1m_reused);
}
- clear_bit(CLEAN_LIST_BUSY_BIT, flag);
- preempt_enable();
return ibmr;
}
-static inline void wait_clean_list_grace(void)
-{
- int cpu;
- unsigned long *flag;
-
- for_each_online_cpu(cpu) {
- flag = &per_cpu(clean_list_grace, cpu);
- while (test_bit(CLEAN_LIST_BUSY_BIT, flag))
- cpu_relax();
- }
-}
-
void rds_ib_sync_mr(void *trans_private, int direction)
{
struct rds_ib_mr *ibmr = trans_private;
@@ -324,8 +306,7 @@ static unsigned int llist_append_to_list(struct llist_head *llist,
* of clusters. Each cluster has linked llist nodes of
* MR_CLUSTER_SIZE mrs that are ready for reuse.
*/
-static void list_to_llist_nodes(struct rds_ib_mr_pool *pool,
- struct list_head *list,
+static void list_to_llist_nodes(struct list_head *list,
struct llist_node **nodes_head,
struct llist_node **nodes_tail)
{
@@ -402,8 +383,13 @@ int rds_ib_flush_mr_pool(struct rds_ib_mr_pool *pool,
*/
dirty_to_clean = llist_append_to_list(&pool->drop_list, &unmap_list);
dirty_to_clean += llist_append_to_list(&pool->free_list, &unmap_list);
- if (free_all)
+ if (free_all) {
+ unsigned long flags;
+
+ spin_lock_irqsave(&pool->clean_lock, flags);
llist_append_to_list(&pool->clean_list, &unmap_list);
+ spin_unlock_irqrestore(&pool->clean_lock, flags);
+ }
free_goal = rds_ib_flush_goal(pool, free_all);
@@ -416,27 +402,20 @@ int rds_ib_flush_mr_pool(struct rds_ib_mr_pool *pool,
rds_ib_unreg_fmr(&unmap_list, &nfreed, &unpinned, free_goal);
if (!list_empty(&unmap_list)) {
- /* we have to make sure that none of the things we're about
- * to put on the clean list would race with other cpus trying
- * to pull items off. The llist would explode if we managed to
- * remove something from the clean list and then add it back again
- * while another CPU was spinning on that same item in llist_del_first.
- *
- * This is pretty unlikely, but just in case wait for an llist grace period
- * here before adding anything back into the clean list.
- */
- wait_clean_list_grace();
-
- list_to_llist_nodes(pool, &unmap_list, &clean_nodes, &clean_tail);
+ unsigned long flags;
+
+ list_to_llist_nodes(&unmap_list, &clean_nodes, &clean_tail);
if (ibmr_ret) {
*ibmr_ret = llist_entry(clean_nodes, struct rds_ib_mr, llnode);
clean_nodes = clean_nodes->next;
}
/* more than one entry in llist nodes */
- if (clean_nodes)
+ if (clean_nodes) {
+ spin_lock_irqsave(&pool->clean_lock, flags);
llist_add_batch(clean_nodes, clean_tail,
&pool->clean_list);
-
+ spin_unlock_irqrestore(&pool->clean_lock, flags);
+ }
}
atomic_sub(unpinned, &pool->free_pinned);
@@ -610,6 +589,7 @@ struct rds_ib_mr_pool *rds_ib_create_mr_pool(struct rds_ib_device *rds_ibdev,
init_llist_head(&pool->free_list);
init_llist_head(&pool->drop_list);
init_llist_head(&pool->clean_list);
+ spin_lock_init(&pool->clean_lock);
mutex_init(&pool->flush_lock);
init_waitqueue_head(&pool->flush_wait);
INIT_DELAYED_WORK(&pool->flush_worker, rds_ib_mr_pool_flush_worker);
--
2.22.0
