linux-kernel - Re: [PATCH] perf/core: remove a meaningless pair of rcu_read

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Date:   Wed, 8 Jun 2022 11:43:13 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Yanfei Xu <yanfei.xu@...el.com>
Cc:     mingo@...hat.com, acme@...nel.org, mark.rutland@....com,
        alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
        namhyung@...nel.org, linux-kernel@...r.kernel.org,
        linux-perf-users@...r.kernel.org
Subject: Re: [PATCH] perf/core: remove a meaningless pair of
 rcu_read_{,un}lock()

On Wed, Jun 08, 2022 at 05:09:38PM +0800, Yanfei Xu wrote:
> Per the codes, this pair of rcu_read_{,un}lock() protects nothing.
> Let's remove it.
> 
> Further, the variable "pmu" is safe as it is in the SRCU read-side
> critical scope of "pmus_srcu" and perf_pmu_unregister() delete it
> after calling synchronize_srcu.
> 
> Signed-off-by: Yanfei Xu <yanfei.xu@...el.com>
> ---
>  kernel/events/core.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 950b25c3f210..36b0df6feab4 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -11296,9 +11296,7 @@ static struct pmu *perf_init_event(struct perf_event *event)
>  	}
>  
>  again:
> -	rcu_read_lock();
>  	pmu = idr_find(&pmu_idr, type);
> -	rcu_read_unlock();

You're mistaken, this is required for the radix tree internal nodes.
Without it we can't safely traverse the radix tree without full
serialization against the modifiers.