Re: [PATCH v33 5/7] sched/proxy: Break cyclic proxy chains by deactivating blocked tasks

[Kayra Cizmeci <kayracizmeci@gmail.com>]

On Fri, Oct 23, 2026 at 10:04:16PM +0000, John Stultz wrote:
> From: "zhidao su (Xiaomi)" <soolaugust@gmail.com>
> 
> find_proxy_task() follows the blocked_on/owner chain for a
> blocked donor. If the chain forms a cycle, __schedule() can keep
> walking the same chain while holding rq->lock.
> Signed-off-by: zhidao su (Xiaomi) <soolaugust@gmail.com>
> [jstultz: Simplified slightly, dropping WARN_ON and owner == donor
>  check, slightly reworked commit message]
> Signed-off-by: John Stultz <jstultz@google.com>
> ---
>  kernel/sched/core.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 43ba76b5a3464..12a42252bf474 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6984,6 +6984,8 @@ static void proxy_migrate_task(struct rq *rq, struct rq_flags *rf,
>  	proxy_reacquire_rq_lock(rq, rf);
>  }
>  
> +#define MAX_PROXY_CHAIN_DEPTH 1024
> +
>  /*
>   * Find runnable lock owner to proxy for mutex blocked donor
>   *
> @@ -7019,6 +7021,7 @@ find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
>  	int this_cpu = cpu_of(rq);
>  	struct task_struct *p;
>  	int owner_cpu;
> +	int chain_depth = 0;
>  
>  	/* Follow blocked_on chain. */
>  	for (p = donor; p->is_blocked; p = owner) {
> @@ -7139,6 +7142,13 @@ find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
>  			 */
>  			goto resched_idle;
>  		}
> +
> +		/* Limit the depth we'll proxy to avoid cyclic deadlocks */
> +		if (++chain_depth > MAX_PROXY_CHAIN_DEPTH) {
> +			__clear_task_blocked_on(p, NULL);
> +			goto deactivate;
> +		}
> +
>  		/*
>  		 * OK, now we're absolutely sure @owner is on this
>  		 * rq, therefore holding @rq->lock is sufficient to
> -- 
> 2.56.0.257.gf7497c7d27-goog
> 

This needs a Fixes tag, no?

Fixes: 7de9d4f94638 ("sched: Start blocked_on chain processing in find_proxy_task()")