Re: [v7 PATCH 2/2] hung_task: Enable runtime reset of hung_task_detect_count

[Petr Mladek]

On Sun 2026-01-25 08:58:48, Aaron Tomlin wrote:
> Currently, the hung_task_detect_count sysctl provides a cumulative count
> of hung tasks since boot. In long-running, high-availability
> environments, this counter may lose its utility if it cannot be reset
> once an incident has been resolved. Furthermore, the previous
> implementation relied upon implicit ordering, which could not strictly
> guarantee that diagnostic metadata published by one CPU was visible to
> the panic logic on another.
> 
> This patch introduces the capability to reset the detection count by
> writing "0" to the hung_task_detect_count sysctl. The proc_handler logic
> has been updated to validate this input and atomically reset the
> counter.
> 
> The synchronisation of sysctl_hung_task_detect_count relies upon a
> transactional model to ensure the integrity of the detection counter
> against concurrent resets from userspace. The application of
> atomic_long_read_acquire() and atomic_long_cmpxchg_release() is correct
> and provides the following guarantees:
> 
>     1. Prevention of Load-Store Reordering via Acquire Semantics By
>        utilising atomic_long_read_acquire() to snapshot the counter
>        before initiating the task traversal, we establish a strict
>        memory barrier. This prevents the compiler or hardware from
>        reordering the initial load to a point later in the scan. Without
>        this "acquire" barrier, a delayed load could potentially read a
>        "0" value resulting from a userspace reset that occurred
>        mid-scan. This would lead to the subsequent cmpxchg succeeding
>        erroneously, thereby overwriting the user's reset with stale
>        increment data.
> 
>     2. Atomicity of the "Commit" Phase via Release Semantics The
>        atomic_long_cmpxchg_release() serves as the transaction's commit
>        point. The "release" barrier ensures that all diagnostic
>        recordings and task-state observations made during the scan are
>        globally visible before the counter is incremented.
> 
>     3. Race Condition Resolution This pairing effectively detects any
>        "out-of-band" reset of the counter. If
>        sysctl_hung_task_detect_count is modified via the procfs
>        interface during the scan, the final cmpxchg will detect the
>        discrepancy between the current value and the "acquire" snapshot.
>        Consequently, the update will fail, ensuring that a reset command
>        from the administrator is prioritised over a scan that may have
>        been invalidated by that very reset.
> 
> Signed-off-by: Aaron Tomlin <atomlin@xxxxxxxxxxx>

LGTM, feel free to use:

Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>

Best Regards,
Petr