Re: [PATCH for-next-fixes v2 5/5] mm/slab: serialize deferred_work_barrier()

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 11/3/26 15:47, Harry Yoo (Meta) wrote:
> irq_work_sync() uses rcuwait instead of busy waiting in two cases:
> 
>   1. The kernel is using PREEMPT_RT and the irq work does not run in a
>      hardirq context.
> 
>   2. The architecture cannot send inter-processor interrupts to make
>      busy waiting reasonably short.
> 
> However, rcuwait.h says:
>> The caller is responsible for locking around rcuwait_wait_event(),
>> and [prepare_to/finish]_rcuwait() such that writes to @task are
>> properly serialized.
> 
> Since deferred_work_barrier() calls irq_work_sync() without any locks,
> it can potentially cause a hang as writes to @task are not serialized.
> 
> Fix this by serializing deferred_work_barrier() with a dedicated mutex.
> We cannot reuse slab_mutex because e.g.) some users of
> kvfree_rcu_barrier() are outside mm/.
> 
> Reported-by: Sashiko <sashiko-bot@kernel.org>
> Closes: https://sashiko.dev/#/patchset/20260615-kfree_rcu_nolock-v3-0-70a54f3775bb%40kernel.org?part=5
> Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and kfree_nolock().")
> Signed-off-by: Harry Yoo (Meta) <harry@kernel.org>

Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>


> ---
>  mm/slub.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 6dfa8ca3c937..50e88edcef3a 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -6636,8 +6636,12 @@ void defer_kfree_rcu(struct kvfree_rcu_head *head)
>  /* Must be called before flush_rcu_sheaves_on_cache() */
>  void deferred_work_barrier(void)
>  {
> +	static DEFINE_MUTEX(barrier_mutex);
>  	int cpu;
>  
> +	/* irq_work_sync() may use rcuwait that requires serialization */
> +	guard(mutex)(&barrier_mutex);
> +
>  	for_each_possible_cpu(cpu)
>  		irq_work_sync(&per_cpu_ptr(&deferred_percpu_work, cpu)->work);
>  }
>