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

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