linux-kernel - Re: [PATCH v1 2/2] x86, perf,cqm: handle CPU hotplug

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Message-ID: <alpine.DEB.2.11.1506182146160.4107@nanos>
Date:	Thu, 18 Jun 2015 21:47:36 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Kanaka Juvva <kanaka.d.juvva@...ux.intel.com>
cc:	kanaka.d.juvva@...el.com, glenn.p.williamson@...el.com,
	matt.fleming@...el.com, will.auld@...el.com, andi@...stfloor.org,
	ananth.s.narayan@...el.com, linux-kernel@...r.kernel.org,
	andrew.j.herdrich@...el.com, tony.luck@...el.com,
	peterz@...radead.org, x86@...nel.org, mingo@...hat.com,
	hpa@...or.com, luto@...capital.net, dvlasenk@...hat.com,
	bp@...en8.de, dave.hansen@...ux.intel.com,
	peter.p.waskiewicz.jr@...el.com, imammedo@...hat.com, bp@...e.de,
	ross.zwisler@...ux.intel.com, jacob.w.shin@...il.com,
	dirk.j.brandewie@...el.com, vikas.shivappa@...el.com,
	edwin.verplanke@...el.com, tomasz.kantecki@...el.com
Subject: Re: [PATCH v1 2/2] x86, perf,cqm: handle CPU hotplug

On Thu, 18 Jun 2015, Kanaka Juvva wrote:

> Added lock in event reader function. The cqm_pick_event_reader() function
> accesses cqm_cpumask and it is critical section between this and
> cqm_stable().
> 
> This situation is true when a CPU is hotplugged. Mutex is used to protect
> the critical section.
> 
> Signed-off-by: Kanaka Juvva <kanaka.d.juvva@...ux.intel.com>
> ---
>  arch/x86/kernel/cpu/perf_event_intel_cqm.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/perf_event_intel_cqm.c b/arch/x86/kernel/cpu/perf_event_intel_cqm.c
> index 1880761..e17e37f 100644
> --- a/arch/x86/kernel/cpu/perf_event_intel_cqm.c
> +++ b/arch/x86/kernel/cpu/perf_event_intel_cqm.c
> @@ -1239,12 +1239,15 @@ static inline void cqm_pick_event_reader(int cpu)
>  	int phys_id = topology_physical_package_id(cpu);
>  	int i;
>  
> +	mutex_lock(&cache_mutex);

I already explained it to Vikas. You CANNOT take a mutex in that code
path as it runs with interrupts disabled on a CPU which cannot
schedule.

Sigh.

	tglx
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