Re: [PATCH v9 1/2] hung_task: Reset warning budget when problem gets resolved

[Petr Mladek <pmladek@suse.com>]

On Thu 2026-08-27 23:30:01, Lance Yang wrote:
> On Wed, Aug 26, 2026 at 01:17:47PM +0200, Petr Mladek wrote:
> >> @@ -314,6 +317,11 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
> >>  	if (test_taint(TAINT_DIE) || did_panic)
> >>  		return;
> >>  
> >> +	if (atomic_xchg(&reset_hung_task_warnings, 0))
> >
> >I would use here atomic_xchg_acquire(). It serializes the ordering
> >of reset_hung_task_warnings vs sysctl_hung_task_warnings.
> >It would make it symetric with the barrier in the sysctl handler.
> 
> Yep, _acquire is enough here. Plain atomic_xchg() is already fully
> ordered, though, so this looks like making the intent clearer rather
> than fixing the ordering :)

Yes, my intention was to make the ordering more clear and symmetric.
The original code worked because the barrier was even stronger.

> The old-value return already makes plain atomic_xchg() fully ordered :)
> 
> ORDERING  (see memory-barriers.txt)
> --------
> 
> The rule of thumb:
> ...
> - RMW operations that have a return value are fully ordered;
> ...
> Except of course when a successful operation has an explicit ordering
> like:
> 
> {}_relaxed: unordered
> {}_acquire: the R of the RMW (or atomic_read) is an ACQUIRE
> {}_release: the W of the RMW (or atomic_set)  is a  RELEASE
> 
> >
> >> +		hung_task_warnings_printed =
> >> +			READ_ONCE(sysctl_hung_task_warnings);
> >
> >This would work only when "sysctl_hung_task_warnings"
> >is updated using WRITE_ONCE(). But it seems that this
> >is not the case. My understading is that it is updated by:
> 
> Wait, I think proc_dointvec_minmax() already handles this.

You are right.

> For proc_dointvec_minmax(), the converter is:
> 
> int proc_dointvec_minmax(const struct ctl_table *table, int dir,
> 		  void *buffer, size_t *lenp, loff_t *ppos)
> {
> 	return do_proc_dointvec(table, dir, buffer, lenp, ppos,
> 				do_proc_int_conv_minmax);
> }
> 
> Here, i is table->data, while lval is local:

I have missed this.

> static int do_proc_dointvec(const struct ctl_table *table, int dir,
> 		  void *buffer, size_t *lenp, loff_t *ppos,
> 		  int (*conv)(bool *negp, unsigned long *u_ptr, int *k_ptr,
> 			      int dir, const struct ctl_table *table))
> {
> ...
> 	i = (int *) table->data;
> 	vleft = table->maxlen / sizeof(*i);
> ...
> 	for (; left && vleft--; i++, first=0) {
> 		unsigned long lval;
> 		bool neg;
> 
> 		if (SYSCTL_USER_TO_KERN(dir)) {
> 			proc_skip_spaces(&p, &left);
> 
> 			if (!left)
> 				break;
> 			err = proc_get_long(&p, &left, &lval, &neg,
> 					     proc_wspace_sep,
> 					     sizeof(proc_wspace_sep), NULL);

I have missed that proc_get_long() assigns the value to the local
variable @lval.

> 			if (err)
> 				break;
> 			if (conv(&neg, &lval, i, 1, table)) {
> 				err = -EINVAL;
> 				break;

The real asigment to table->data is done here. And I agree that it
goes down to proc_int_conv() which does WRITE_ONCE().

So, we are on the safe side and do _not_ need the proxy table.

Now, I am not sure whether we need v10. It might be worth it.
AFAIK, Andrew has not taken this patchset yet...

I am sorry for complications.

Best Regards,
Petr