linux-kernel - Re: [4.9-rc1+] intel_uncore builtin + CONFIG_DEBUG_TEST_DRIVER

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Message-ID: <20161020053944.GQ3102@twins.programming.kicks-ass.net>
Date:   Thu, 20 Oct 2016 07:39:44 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     CAI Qian <caiqian@...hat.com>, Rob Herring <robh@...nel.org>,
        Kan Liang <kan.liang@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [4.9-rc1+] intel_uncore builtin +
 CONFIG_DEBUG_TEST_DRIVER_REMOVE kernel panic

On Wed, Oct 19, 2016 at 09:19:43PM +0200, Jiri Olsa wrote:
> I think the reason here is that presume pmu devices are always added,
> but we add them only if pmu_bus_running (in perf_event_sysfs_init)
> is set which might happen after uncore initcall
> 
> attached patch fixes the issue for me

Right, we never expected to be unloaded before userspace runs.

Strictly speaking we should only read pmu_bus_running while holding
pmus_lock, that way we're serialized against perf_event_sysfs_init()
flipping it while we're being removed etc..

With the current setup the introduced race is harmless, but who knows
what other crazy these device people will come up with ;-)

> ---
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index c6e47e97b33f..c2099b799d16 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -8871,8 +8871,10 @@ void perf_pmu_unregister(struct pmu *pmu)
>  		idr_remove(&pmu_idr, pmu->type);
>  	if (pmu->nr_addr_filters)
>  		device_remove_file(pmu->dev, &dev_attr_nr_addr_filters);
> -	device_del(pmu->dev);
> -	put_device(pmu->dev);
> +	if (pmu_bus_running) {
> +		device_del(pmu->dev);
> +		put_device(pmu->dev);
> +	}
>  	free_pmu_context(pmu);
>  }
>  EXPORT_SYMBOL_GPL(perf_pmu_unregister);