On Wed, Mar 04, 2026 at 05:37:31PM +0800, Yafang Shao wrote:
> On Wed, Mar 4, 2026 at 5:03 PM Peter Zijlstra <[email protected]> wrote:
> >
> > On Wed, Mar 04, 2026 at 03:46:49PM +0800, Yafang Shao wrote:
> > > Introduce mutex_lock_nospin(), a helper that disables optimistic spinning
> > > on the owner for specific heavy locks. This prevents long spinning times
> > > that can lead to latency spikes for other tasks on the same runqueue.
> >
> > This makes no sense; spinning stops on need_resched().
>
> Hello Peter,
>
> The condition to stop spinning on need_resched() relies on the mutex
> owner remaining unchanged. However, when multiple tasks contend for
> the same lock, the owner can change frequently. This creates a
> potential TOCTOU (Time of Check to Time of Use) issue.
>
> mutex_optimistic_spin
> owner = __mutex_trylock_or_owner(lock);
> mutex_spin_on_owner
> // the __mutex_owner(lock) might get a new owner.
> while (__mutex_owner(lock) == owner)
>
How do these new owners become the owner? Are they succeeding the
__mutex_trylock() that sits before mutex_optimistic_spin() and
effectively starving the spinner?
Something like the below would make a difference if that were so.
---
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index c867f6c15530..0796e77a8c3b 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -521,7 +521,7 @@ static __always_inline bool
mutex_optimistic_spin(struct mutex *lock, struct ww_acquire_ctx *ww_ctx,
struct mutex_waiter *waiter)
{
- return false;
+ return __mutex_trylock(lock);
}
#endif
@@ -614,8 +614,7 @@ __mutex_lock_common(struct mutex *lock, unsigned int state,
unsigned int subclas
mutex_acquire_nest(&lock->dep_map, subclass, 0, nest_lock, ip);
trace_contention_begin(lock, LCB_F_MUTEX | LCB_F_SPIN);
- if (__mutex_trylock(lock) ||
- mutex_optimistic_spin(lock, ww_ctx, NULL)) {
+ if (mutex_optimistic_spin(lock, ww_ctx, NULL)) {
/* got the lock, yay! */
lock_acquired(&lock->dep_map, ip);
if (ww_ctx)
