linux-kernel - Re: [tip:smp/hotplug] perf: Cure hotplug lock ordering issues

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Date:   Mon, 24 Apr 2017 11:04:42 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...nel.org,
        hpa@...or.com
Cc:     linux-tip-commits@...r.kernel.org
Subject: Re: [tip:smp/hotplug] perf: Cure hotplug lock ordering issues

On Sun, Apr 23, 2017 at 10:16:34AM -0700, tip-bot for Thomas Gleixner wrote:
> Commit-ID:  14aff62ede39a7b069beb7b95c1d02edb8ee912d
> Gitweb:     http://git.kernel.org/tip/14aff62ede39a7b069beb7b95c1d02edb8ee912d
> Author:     Thomas Gleixner <tglx@...utronix.de>
> AuthorDate: Sun, 23 Apr 2017 12:13:09 +0200
> Committer:  Thomas Gleixner <tglx@...utronix.de>
> CommitDate: Sun, 23 Apr 2017 19:11:27 +0200
> 
> perf: Cure hotplug lock ordering issues
> 
> The get_online_cpus() rework unearthed lock ordering issues.
> 
> Reorder hotpluglock and perf_sched_mutex() to avoid circular locking
> dependencies and convert static_branch_disable() to the cpuslocked version
> to avoid hotplug lock recursion.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Cc: Peter Zijlstra <peterz@...radead.org>
> ---
>  kernel/events/core.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 8aa3063..b5b4f52f 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -4016,10 +4016,12 @@ static void unaccount_event(struct perf_event *event)
>  
>  static void perf_sched_delayed(struct work_struct *work)
>  {
> +	get_online_cpus();
>  	mutex_lock(&perf_sched_mutex);
>  	if (atomic_dec_and_test(&perf_sched_count))
> -		static_branch_disable(&perf_sched_events);
> +		static_branch_disable_cpuslocked(&perf_sched_events);
>  	mutex_unlock(&perf_sched_mutex);
> +	put_online_cpus();
>  }

So I think we can simply remove the perf_sched_mutex here. Its only
purpose is to serialize the 0->1 state, such that nobody passes before
we've done the additional sync_sched() call.

Once we're >=1, it really doesn't matter anymore. Not sure why I put it
in here. Maybe extensive paranoia..