Re: [PATCH v2] coresight: Serialize enabling/disabling a link device.

[Suzuki K Poulose <suzuki.poulose@arm.com>]

Hi Yabin,

On 09/08/2019 22:45, Yabin Cui wrote:
> When tracing etm data of multiple threads on multiple cpus through perf
> interface, some link devices are shared between paths of different cpus.
> It creates race conditions when different cpus wants to enable/disable
> the same link device at the same time.
> 
> Example 1:
> Two cpus want to enable different ports of a coresight funnel, thus
> calling the funnel enable operation at the same time. But the funnel
> enable operation isn't reentrantable.
> 
> Example 2:
> For an enabled coresight dynamic replicator with refcnt=1, one cpu wants
> to disable it, while another cpu wants to enable it. Ideally we still have
> an enabled replicator with refcnt=1 at the end. But in reality the result
> is uncertain.
> 
> Since coresight devices claim themselves when enabled for self-hosted
> usage, the race conditions above usually make the link devices not usable
> after many cycles.
> 
> To fix the race conditions, this patch adds a spinlock to serialize
> enabling/disabling a link device.
> 
> Signed-off-by: Yabin Cui <yabinc@google.com>
> ---
> 
> v1 -> v2: extend lock range to protect read of refcnt in
> coresight_disable_link().

Thanks for this. Please find my comments below.

> 
> ---
>   drivers/hwtracing/coresight/coresight.c | 12 +++++++++++-
>   include/linux/coresight.h               |  3 +++
>   2 files changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/hwtracing/coresight/coresight.c b/drivers/hwtracing/coresight/coresight.c
> index 55db77f6410b..e526bdeaeb22 100644
> --- a/drivers/hwtracing/coresight/coresight.c
> +++ b/drivers/hwtracing/coresight/coresight.c
> @@ -256,6 +256,7 @@ static int coresight_enable_link(struct coresight_device *csdev,
>   	int ret;
>   	int link_subtype;
>   	int refport, inport, outport;
> +	unsigned long flags;
>   
>   	if (!parent || !child)
>   		return -EINVAL;
> @@ -274,15 +275,18 @@ static int coresight_enable_link(struct coresight_device *csdev,
>   	if (refport < 0)
>   		return refport;
>   
> +	spin_lock_irqsave(&csdev->spinlock, flags);
>   	if (atomic_inc_return(&csdev->refcnt[refport]) == 1) {
>   		if (link_ops(csdev)->enable) {
>   			ret = link_ops(csdev)->enable(csdev, inport, outport);
>   			if (ret) {
>   				atomic_dec(&csdev->refcnt[refport]);
> +				spin_unlock_irqrestore(&csdev->spinlock, flags);
>   				return ret;
>   			}
>   		}
>   	}
> +	spin_unlock_irqrestore(&csdev->spinlock, flags);
>   
>   	csdev->enable = true;

Please could we move this inside the spin_lock () too ?

>   
> @@ -296,6 +300,7 @@ static void coresight_disable_link(struct coresight_device *csdev,
>   	int i, nr_conns;
>   	int link_subtype;
>   	int refport, inport, outport;
> +	unsigned long flags;
>   
>   	if (!parent || !child)
>   		return;
> @@ -315,14 +320,18 @@ static void coresight_disable_link(struct coresight_device *csdev,
>   		nr_conns = 1;
>   	}
>   
> +	spin_lock_irqsave(&csdev->spinlock, flags);
>   	if (atomic_dec_return(&csdev->refcnt[refport]) == 0) {
>   		if (link_ops(csdev)->disable)
>   			link_ops(csdev)->disable(csdev, inport, outport);
>   	}
>   
>   	for (i = 0; i < nr_conns; i++)
> -		if (atomic_read(&csdev->refcnt[i]) != 0)
> +		if (atomic_read(&csdev->refcnt[i]) != 0) {
> +			spin_unlock_irqrestore(&csdev->spinlock, flags);
>   			return;
> +		}
> +	spin_unlock_irqrestore(&csdev->spinlock, flags);
>   
>   	csdev->enable = false;
>   }

And this too ? I understand this may not be used right now, but we can avoid any
surprises when we do so.

With the above fixed, :

Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>