Re: Patch "sched_ext: Fix missing rq lock in scx_bpf_cpuperf_set()" has been added to the 6.12-stable tree

[Andrea Righi <arighi@nvidia.com>]

Hi,

On Sun, May 18, 2025 at 06:38:01AM -0400, Sasha Levin wrote:
> This is a note to let you know that I've just added the patch titled
> 
>     sched_ext: Fix missing rq lock in scx_bpf_cpuperf_set()
> 
> to the 6.12-stable tree which can be found at:
>     http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
> 
> The filename of the patch is:
>      sched_ext-fix-missing-rq-lock-in-scx_bpf_cpuperf_set.patch
> and it can be found in the queue-6.12 subdirectory.
> 
> If you, or anyone else, feels it should not be added to the stable tree,
> please let <stable@vger.kernel.org> know about it.

This requires upstream commit 18853ba782bef ("sched_ext: Track currently
locked rq").

Thanks,
-Andrea

> 
> 
> 
> commit b19ccb5dc4d3d35b83b94fe2514456dc0b3a9ba5
> Author: Andrea Righi <arighi@nvidia.com>
> Date:   Tue Apr 22 10:26:33 2025 +0200
> 
>     sched_ext: Fix missing rq lock in scx_bpf_cpuperf_set()
>     
>     [ Upstream commit a11d6784d7316a6c77ca9f14fb1a698ebbb3c1fb ]
>     
>     scx_bpf_cpuperf_set() can be used to set a performance target level on
>     any CPU. However, it doesn't correctly acquire the corresponding rq
>     lock, which may lead to unsafe behavior and trigger the following
>     warning, due to the lockdep_assert_rq_held() check:
>     
>     [   51.713737] WARNING: CPU: 3 PID: 3899 at kernel/sched/sched.h:1512 scx_bpf_cpuperf_set+0x1a0/0x1e0
>     ...
>     [   51.713836] Call trace:
>     [   51.713837]  scx_bpf_cpuperf_set+0x1a0/0x1e0 (P)
>     [   51.713839]  bpf_prog_62d35beb9301601f_bpfland_init+0x168/0x440
>     [   51.713841]  bpf__sched_ext_ops_init+0x54/0x8c
>     [   51.713843]  scx_ops_enable.constprop.0+0x2c0/0x10f0
>     [   51.713845]  bpf_scx_reg+0x18/0x30
>     [   51.713847]  bpf_struct_ops_link_create+0x154/0x1b0
>     [   51.713849]  __sys_bpf+0x1934/0x22a0
>     
>     Fix by properly acquiring the rq lock when possible or raising an error
>     if we try to operate on a CPU that is not the one currently locked.
>     
>     Fixes: d86adb4fc0655 ("sched_ext: Add cpuperf support")
>     Signed-off-by: Andrea Righi <arighi@nvidia.com>
>     Acked-by: Changwoo Min <changwoo@igalia.com>
>     Signed-off-by: Tejun Heo <tj@kernel.org>
>     Signed-off-by: Sasha Levin <sashal@kernel.org>
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 7ed25654820fd..0147c4452f4df 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -7018,13 +7018,32 @@ __bpf_kfunc void scx_bpf_cpuperf_set(s32 cpu, u32 perf)
>  	}
>  
>  	if (ops_cpu_valid(cpu, NULL)) {
> -		struct rq *rq = cpu_rq(cpu);
> +		struct rq *rq = cpu_rq(cpu), *locked_rq = scx_locked_rq();
> +		struct rq_flags rf;
> +
> +		/*
> +		 * When called with an rq lock held, restrict the operation
> +		 * to the corresponding CPU to prevent ABBA deadlocks.
> +		 */
> +		if (locked_rq && rq != locked_rq) {
> +			scx_ops_error("Invalid target CPU %d", cpu);
> +			return;
> +		}
> +
> +		/*
> +		 * If no rq lock is held, allow to operate on any CPU by
> +		 * acquiring the corresponding rq lock.
> +		 */
> +		if (!locked_rq) {
> +			rq_lock_irqsave(rq, &rf);
> +			update_rq_clock(rq);
> +		}
>  
>  		rq->scx.cpuperf_target = perf;
> +		cpufreq_update_util(rq, 0);
>  
> -		rcu_read_lock_sched_notrace();
> -		cpufreq_update_util(cpu_rq(cpu), 0);
> -		rcu_read_unlock_sched_notrace();
> +		if (!locked_rq)
> +			rq_unlock_irqrestore(rq, &rf);
>  	}
>  }
>