[PATCH 1/2] locking/mutex: Don't hog RCU read lock while optimistically spinning

[Sultan Alsawaf]

From: Sultan Alsawaf <sul...@kerneltoast.com>

There's no reason to hold an RCU read lock the entire time while
optimistically spinning for a mutex lock. This can needlessly lengthen
RCU grace periods and slow down synchronize_rcu() when it doesn't brute
force the RCU grace period via rcupdate.rcu_expedited=1.

Signed-off-by: Sultan Alsawaf <sul...@kerneltoast.com>
---
 kernel/locking/mutex.c | 25 +++++++++++++++++--------
 1 file changed, 17 insertions(+), 8 deletions(-)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 5352ce50a97e..cc5676712458 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -552,21 +552,31 @@ bool mutex_spin_on_owner(struct mutex *lock, struct 
task_struct *owner,
 {
        bool ret = true;
 
-       rcu_read_lock();
-       while (__mutex_owner(lock) == owner) {
+       for (;;) {
+               unsigned int cpu;
+               bool same_owner;
+
                /*
-                * Ensure we emit the owner->on_cpu, dereference _after_
-                * checking lock->owner still matches owner. If that fails,
+                * Ensure lock->owner still matches owner. If that fails,
                 * owner might point to freed memory. If it still matches,
                 * the rcu_read_lock() ensures the memory stays valid.
                 */
-               barrier();
+               rcu_read_lock();
+               same_owner = __mutex_owner(lock) == owner;
+               if (same_owner) {
+                       ret = owner->on_cpu;
+                       if (ret)
+                               cpu = task_cpu(owner);
+               }
+               rcu_read_unlock();
+
+               if (!ret || !same_owner)
+                       break;
 
                /*
                 * Use vcpu_is_preempted to detect lock holder preemption issue.
                 */
-               if (!owner->on_cpu || need_resched() ||
-                               vcpu_is_preempted(task_cpu(owner))) {
+               if (need_resched() || vcpu_is_preempted(cpu)) {
                        ret = false;
                        break;
                }
@@ -578,7 +588,6 @@ bool mutex_spin_on_owner(struct mutex *lock, struct 
task_struct *owner,
 
                cpu_relax();
        }
-       rcu_read_unlock();
 
        return ret;
 }
-- 
2.28.0