Re: [PATCH 1/2] sched_ext: Prevent SCX_KICK_WAIT deadlock by serialization

[Andrea Righi <arighi@nvidia.com>]

Hi Christian,

On Mon, Mar 16, 2026 at 10:02:48AM +0000, Christian Loehle wrote:
> SCX_KICK_WAIT causes kick_cpus_irq_workfn() to busy-wait using
> smp_cond_load_acquire() until the target CPU's current SCX task has been
> context-switched out (its kick_sync counter advanced).
> 
> If multiple CPUs each issue SCX_KICK_WAIT targeting one another
> concurrently — e.g. CPU A waits for CPU B, B waits for CPU C, C waits for
> CPU A — all CPUs can end up wedged inside smp_cond_load_acquire()
> simultaneously.  Because each victim CPU is spinning in hardirq/irq_work
> context, it cannot reschedule, so no kick_sync counter ever advances and
> the system deadlocks.
> 
> Fix this by serializing access to the wait loop behind a global raw
> spinlock (scx_kick_wait_lock).  Only one CPU at a time may execute the
> wait loop; any other CPU that has SCX_KICK_WAIT work to do and fails to
> acquire the lock records itself in scx_kick_wait_pending and returns.
> When the active waiter finishes and releases the lock, it replays the
> pending set by re-queuing each pending CPU's kick_cpus_irq_work, ensuring
> no wait request is silently dropped.
> 
> This is deliberately a coarse serialization: multiple simultaneous wait
> operations now run sequentially, increasing latency.  In exchange,
> deadlocks are impossible regardless of the cycle length (A->B->C->...->A).
> 
> Also clear scx_kick_wait_pending in free_kick_syncs() so that any stale
> bits left by a CPU that deferred just as the scheduler exited are reset
> before the next scheduler instance loads.
> 
> Fixes: 90e55164dad4 ("sched_ext: Implement SCX_KICK_WAIT")
> Signed-off-by: Christian Loehle <christian.loehle@arm.com>
> ---
>  kernel/sched/ext.c | 45 +++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 43 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 26a6ac2f8826..b63ae13d0486 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -89,6 +89,19 @@ struct scx_kick_syncs {
>  
>  static DEFINE_PER_CPU(struct scx_kick_syncs __rcu *, scx_kick_syncs);
>  
> +/*
> + * Serialize %SCX_KICK_WAIT processing across CPUs to avoid wait cycles.
> + * Callers failing to acquire @scx_kick_wait_lock defer by recording
> + * themselves in @scx_kick_wait_pending and are retriggered when the active
> + * waiter completes.
> + *
> + * Lock ordering: @scx_kick_wait_lock is always acquired before
> + * @scx_kick_wait_pending_lock; the two are never taken in the opposite order.
> + */
> +static DEFINE_RAW_SPINLOCK(scx_kick_wait_lock);
> +static DEFINE_RAW_SPINLOCK(scx_kick_wait_pending_lock);
> +static cpumask_t scx_kick_wait_pending;
> +
>  /*
>   * Direct dispatch marker.
>   *
> @@ -4279,6 +4292,13 @@ static void free_kick_syncs(void)
>  		if (to_free)
>  			kvfree_rcu(to_free, rcu);
>  	}
> +
> +	/*
> +	 * Clear any CPUs that were waiting for the lock when the scheduler
> +	 * exited.  Their irq_work has already returned so no in-flight
> +	 * waiter can observe the stale bits on the next enable.
> +	 */
> +	cpumask_clear(&scx_kick_wait_pending);

Do we need a raw_spin_lock/unlock(&scx_kick_wait_pending_lock) here to make
sure we're not racing with with cpumask_set_cpu()/cpumask_clear_cpu()?
Probably it's not that relevant at this point, but I'd keep the locking for
correctness.

Thanks,
-Andrea

>  }
>  
>  static void scx_disable_workfn(struct kthread_work *work)
> @@ -5647,8 +5667,9 @@ static void kick_cpus_irq_workfn(struct irq_work *irq_work)
>  	struct rq *this_rq = this_rq();
>  	struct scx_rq *this_scx = &this_rq->scx;
>  	struct scx_kick_syncs __rcu *ksyncs_pcpu = __this_cpu_read(scx_kick_syncs);
> -	bool should_wait = false;
> +	bool should_wait = !cpumask_empty(this_scx->cpus_to_wait);
>  	unsigned long *ksyncs;
> +	s32 this_cpu = cpu_of(this_rq);
>  	s32 cpu;
>  
>  	if (unlikely(!ksyncs_pcpu)) {
> @@ -5672,6 +5693,17 @@ static void kick_cpus_irq_workfn(struct irq_work *irq_work)
>  	if (!should_wait)
>  		return;
>  
> +	if (!raw_spin_trylock(&scx_kick_wait_lock)) {
> +		raw_spin_lock(&scx_kick_wait_pending_lock);
> +		cpumask_set_cpu(this_cpu, &scx_kick_wait_pending);
> +		raw_spin_unlock(&scx_kick_wait_pending_lock);
> +		return;
> +	}
> +
> +	raw_spin_lock(&scx_kick_wait_pending_lock);
> +	cpumask_clear_cpu(this_cpu, &scx_kick_wait_pending);
> +	raw_spin_unlock(&scx_kick_wait_pending_lock);
> +
>  	for_each_cpu(cpu, this_scx->cpus_to_wait) {
>  		unsigned long *wait_kick_sync = &cpu_rq(cpu)->scx.kick_sync;
>  
> @@ -5686,11 +5718,20 @@ static void kick_cpus_irq_workfn(struct irq_work *irq_work)
>  		 * task is picked subsequently. The latter is necessary to break
>  		 * the wait when $cpu is taken by a higher sched class.
>  		 */
> -		if (cpu != cpu_of(this_rq))
> +		if (cpu != this_cpu)
>  			smp_cond_load_acquire(wait_kick_sync, VAL != ksyncs[cpu]);
>  
>  		cpumask_clear_cpu(cpu, this_scx->cpus_to_wait);
>  	}
> +
> +	raw_spin_unlock(&scx_kick_wait_lock);
> +
> +	raw_spin_lock(&scx_kick_wait_pending_lock);
> +	for_each_cpu(cpu, &scx_kick_wait_pending) {
> +		cpumask_clear_cpu(cpu, &scx_kick_wait_pending);
> +		irq_work_queue(&cpu_rq(cpu)->scx.kick_cpus_irq_work);
> +	}
> +	raw_spin_unlock(&scx_kick_wait_pending_lock);
>  }
>  
>  /**
> -- 
> 2.34.1
>