Re: [PATCH] sched/core: avoid spurious spinlock recursion splats

[Peter Zijlstra]

On Mon, Feb 05, 2018 at 03:51:18PM +0000, Mark Rutland wrote:
> However, this happens *after* prev->on_cpu is cleared, which allows prev
> to be scheduled on another CPU. If prev then attempts to acquire the
> same rq lock, before the updated rq->lock.owner is made visible, it will
> see itself as the owner.

Cute.

> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index b19552a212de..4f0d2e3701c3 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1342,6 +1342,10 @@ static inline void prepare_lock_switch(struct rq *rq, 
> struct task_struct *next)
>  
>  static inline void finish_lock_switch(struct rq *rq, struct task_struct 
> *prev)
>  {
> +#ifdef CONFIG_DEBUG_SPINLOCK
> +     /* this is a valid case when another task releases the spinlock */
> +     rq->lock.owner = current;
> +#endif
>  #ifdef CONFIG_SMP
>       /*
>        * After ->on_cpu is cleared, the task can be moved to a different CPU.
> @@ -1355,10 +1359,6 @@ static inline void finish_lock_switch(struct rq *rq, 
> struct task_struct *prev)
>        */
>       smp_store_release(&prev->on_cpu, 0);
>  #endif
> -#ifdef CONFIG_DEBUG_SPINLOCK
> -     /* this is a valid case when another task releases the spinlock */
> -     rq->lock.owner = current;
> -#endif
>       /*
>        * If we are tracking spinlock dependencies then we have to
>        * fix up the runqueue lock - which gets 'carried over' from

Right, so patch:

  31cb1bc0dc94 ("sched/core: Rework and clarify prepare_lock_switch()")

munched all that code and the above no longer fits. Does the below
change also work for you? (tip/master)

---
 kernel/sched/core.c | 27 ++++++++++++++++-----------
 1 file changed, 16 insertions(+), 11 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ee420d78e674..abfd10692022 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2600,19 +2600,31 @@ static inline void finish_task(struct task_struct *prev)
 #endif
 }
 
-static inline void finish_lock_switch(struct rq *rq)
+static inline void
+prepare_lock_switch(struct rq *rq, struct task_struct *next, struct rq_flags 
*rf)
 {
+       /*
+        * Since the runqueue lock will be released by the next
+        * task (which is an invalid locking op but in the case
+        * of the scheduler it's an obvious special-case), so we
+        * do an early lockdep release here:
+        */
+       rq_unpin_lock(rq, rf);
+       spin_release(&rq->lock.dep_map, 1, _THIS_IP_);
 #ifdef CONFIG_DEBUG_SPINLOCK
        /* this is a valid case when another task releases the spinlock */
-       rq->lock.owner = current;
+       rq->lock.owner = next;
 #endif
+}
+
+static inline void finish_lock_switch(struct rq *rq)
+{
        /*
         * If we are tracking spinlock dependencies then we have to
         * fix up the runqueue lock - which gets 'carried over' from
         * prev into current:
         */
        spin_acquire(&rq->lock.dep_map, 0, 0, _THIS_IP_);
-
        raw_spin_unlock_irq(&rq->lock);
 }
 
@@ -2843,14 +2855,7 @@ context_switch(struct rq *rq, struct task_struct *prev,
 
        rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP);
 
-       /*
-        * Since the runqueue lock will be released by the next
-        * task (which is an invalid locking op but in the case
-        * of the scheduler it's an obvious special-case), so we
-        * do an early lockdep release here:
-        */
-       rq_unpin_lock(rq, rf);
-       spin_release(&rq->lock.dep_map, 1, _THIS_IP_);
+       prepare_lock_switch(rq, next, rf);
 
        /* Here we just switch the register state and the stack. */
        switch_to(prev, next, prev);