[PATCH v3 01/31] coresight: Change device mode to atomic type

[Leo Yan]

On Thu, Oct 02, 2025 at 01:51:22PM +0100, Suzuki Kuruppassery Poulose wrote:
> On 15/09/2025 11:33, Leo Yan wrote:
> > The device mode is defined as local type. This type cannot promise
> > SMP-safe access.
> > 
> > Change to atomic type and impose relax ordering, which ensures the
> > SMP-safe synchronisation and the ordering between the mode setting and
> > relevant operations.
> > 
> > Fixes: 22fd532eaa0c ("coresight: etm3x: adding operation mode for etm_enable()")
> 
> 
> Do we really have an issue here ? I thought we modify the mode under
> spinlock. Also the fixes commit looks odd. The mode was added to the
> coresight device only recently. Prior to that, it was part of drvdata and
> was serialized by the respective spinlocks. So I don't see how this
> was an issue.

Below is the sequence in which CPU0 enables the path with CPU1 ETM
via sysfs knob, while CPU1 is executing the CPUidle notifier:

  CPU0                                     |  CPU1
                                           |
  coresight_enable_sysfs()                 |
   `> mutex_lock(&coresight_mutex)         |  etm4_cpu_pm_notify()
   `> etm4_enable()                        |   `> etm4_cpu_save()
       `> coresight_take_mode(csdev, mode) |       `> coresight_get_mode(csdev)
       `> Call SMP call                    |
   `> mutex_unlock(&coresight_mutex)       |


(1) The mode is not protected by spinlock. Although the sysfs flow
    acquires a mutex, it does not provide exclusive access to the
    mode.

(2) On SMP systems, the mode is accessed by multiple CPUs. We
    currently use the local_* accessors, which are suitable only for
    per-CPU (local) access, not for SMP access.

If a vairable is not protected by spinlock, and accessed by SMP cores,
then atomic type would be a right choice.

Thanks,
Leo