Re: [v6 PATCH 2/2] hung_task: Enable runtime reset of hung_task_detect_count

[Lance Yang <lance.yang@linux.dev>]

On 2026/1/15 10:32, 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@atomlin.com>
> ---
>   Documentation/admin-guide/sysctl/kernel.rst |   3 +-
>   kernel/hung_task.c                          | 109 +++++++++++++-------
>   2 files changed, 75 insertions(+), 37 deletions(-)
> 
> diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
> index 239da22c4e28..68da4235225a 100644
> --- a/Documentation/admin-guide/sysctl/kernel.rst
> +++ b/Documentation/admin-guide/sysctl/kernel.rst
> @@ -418,7 +418,8 @@ hung_task_detect_count
>   ======================
>   
>   Indicates the total number of tasks that have been detected as hung since
> -the system boot.
> +the system boot or since the counter was reset. The counter is zeroed when
> +a value of 0 is written.
>   
>   This file shows up if ``CONFIG_DETECT_HUNG_TASK`` is enabled.
>   
> diff --git a/kernel/hung_task.c b/kernel/hung_task.c
> index b5ad7a755eb5..2eb9c861bdcc 100644
> --- a/kernel/hung_task.c
> +++ b/kernel/hung_task.c
> @@ -224,24 +224,43 @@ static inline void debug_show_blocker(struct task_struct *task, unsigned long ti
>   }
>   #endif
>   
> -static void check_hung_task(struct task_struct *t, unsigned long timeout,
> -			    unsigned long prev_detect_count)
> +/**
> + * hung_task_diagnostics - Print structured diagnostic info for a hung task.
> + * @t: Pointer to the detected hung task.
> + *
> + * This function consolidates the printing of core diagnostic information
> + * for a task found to be blocked.
> + */
> +static inline void hung_task_diagnostics(struct task_struct *t)
>   {
> -	unsigned long total_hung_task, cur_detect_count;
> -
> -	if (!task_is_hung(t, timeout))
> -		return;
> -
> -	/*
> -	 * This counter tracks the total number of tasks detected as hung
> -	 * since boot.
> -	 */
> -	cur_detect_count = atomic_long_inc_return_relaxed(&sysctl_hung_task_detect_count);
> -	total_hung_task = cur_detect_count - prev_detect_count;
> +	unsigned long blocked_secs = (jiffies - t->last_switch_time) / HZ;
> +
> +	pr_err("INFO: task %s:%d blocked for more than %ld seconds.\n",
> +		t->comm, t->pid, blocked_secs);
> +	pr_err("      %s %s %.*s\n",
> +		print_tainted(), init_utsname()->release,
> +		(int)strcspn(init_utsname()->version, " "),
> +		init_utsname()->version);
> +	if (t->flags & PF_POSTCOREDUMP)
> +		pr_err("      Blocked by coredump.\n");
> +	pr_err("\"echo 0 > /proc/sys/kernel/hung_task_timeout_secs\" disables this message.\n");
> +}

I see hung_task_diagnostics() is still in this patch. I thought
we'd concluded that[1] the refactoring wasn't really necessary for a
single-use block?

[1] 
https://lore.kernel.org/all/noze3vhqjbsuulvvoaw4h5yeinggpwfslrit5vsd2dllfo4ath@qgmp22hoibgn/