Re: [PATCH v3 1/2] coresight: catu: Introduce refcount and spinlock for enabling/disabling

[James Clark <james.clark@linaro.org>]

On 08/04/2025 8:59 pm, Yabin Cui wrote:
> When tracing ETM data on multiple CPUs concurrently via the
> perf interface, the CATU device is shared across different CPU
> paths. This can lead to race conditions when multiple CPUs attempt
> to enable or disable the CATU device simultaneously.
> 
> To address these race conditions, this patch introduces the
> following changes:
> 
> 1. The enable and disable operations for the CATU device are not
>     reentrant. Therefore, a spinlock is added to ensure that only
>     one CPU can enable or disable a given CATU device at any point
>     in time.
> 
> 2. A reference counter is used to manage the enable/disable state
>     of the CATU device. The device is enabled when the first CPU
>     requires it and is only disabled when the last CPU finishes
>     using it. This ensures the device remains active as long as at
>     least one CPU needs it.
> 
> Signed-off-by: Yabin Cui <yabinc@google.com>
> ---
>   drivers/hwtracing/coresight/coresight-catu.c | 25 +++++++++++++-------
>   drivers/hwtracing/coresight/coresight-catu.h |  1 +
>   2 files changed, 18 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-catu.c b/drivers/hwtracing/coresight/coresight-catu.c
> index fa170c966bc3..30b78b2f8adb 100644
> --- a/drivers/hwtracing/coresight/coresight-catu.c
> +++ b/drivers/hwtracing/coresight/coresight-catu.c
> @@ -458,12 +458,17 @@ static int catu_enable_hw(struct catu_drvdata *drvdata, enum cs_mode cs_mode,
>   static int catu_enable(struct coresight_device *csdev, enum cs_mode mode,
>   		       void *data)
>   {
> -	int rc;
> +	int rc = 0;
>   	struct catu_drvdata *catu_drvdata = csdev_to_catu_drvdata(csdev);
> +	guard(raw_spinlock_irqsave)(&catu_drvdata->spinlock);
>   

Very minor nit only because you need to resend anyway, but there should 
be a newline between the variable definitions and the code. Not sure why 
checkpatch doesn't warn here.

> -	CS_UNLOCK(catu_drvdata->base);
> -	rc = catu_enable_hw(catu_drvdata, mode, data);
> -	CS_LOCK(catu_drvdata->base);
> +	if (csdev->refcnt == 0) {
> +		CS_UNLOCK(catu_drvdata->base);
> +		rc = catu_enable_hw(catu_drvdata, mode, data);
> +		CS_LOCK(catu_drvdata->base);
> +	}
> +	if (!rc)
> +		csdev->refcnt++;
>   	return rc;
>   }
>   
> @@ -486,12 +491,15 @@ static int catu_disable_hw(struct catu_drvdata *drvdata)
>   
>   static int catu_disable(struct coresight_device *csdev, void *__unused)
>   {
> -	int rc;
> +	int rc = 0;
>   	struct catu_drvdata *catu_drvdata = csdev_to_catu_drvdata(csdev);
> +	guard(raw_spinlock_irqsave)(&catu_drvdata->spinlock);
>   
> -	CS_UNLOCK(catu_drvdata->base);
> -	rc = catu_disable_hw(catu_drvdata);
> -	CS_LOCK(catu_drvdata->base);
> +	if (--csdev->refcnt == 0) {

Hopefully this never underflows if disable is called again after a 
failed enable. We could add a WARN_ON() but I think this is a general 
case and not specific to these patches so is probably better to do later 
as separate change.

Reviewed-by: James Clark <james.clark@linaro.org>