Re: [RFC PATCH v3 1/8] Use refcount_t for ucounts reference counting

[Alexey Gladkov]

On Tue, Jan 19, 2021 at 07:57:36PM -0600, Eric W. Biederman wrote:
> Alexey Gladkov <gladkov.alexey@xxxxxxxxx> writes:
> 
> > On Mon, Jan 18, 2021 at 12:34:29PM -0800, Linus Torvalds wrote:
> >> On Mon, Jan 18, 2021 at 11:46 AM Alexey Gladkov
> >> <gladkov.alexey@xxxxxxxxx> wrote:
> >> >
> >> > Sorry about that. I thought that this code is not needed when switching
> >> > from int to refcount_t. I was wrong.
> >> 
> >> Well, you _may_ be right. I personally didn't check how the return
> >> value is used.
> >> 
> >> I only reacted to "it certainly _may_ be used, and there is absolutely
> >> no comment anywhere about why it wouldn't matter".
> >
> > I have not found examples where checked the overflow after calling
> > refcount_inc/refcount_add.
> >
> > For example in kernel/fork.c:2298 :
> >
> >    current->signal->nr_threads++;                           
> >    atomic_inc(&current->signal->live);                      
> >    refcount_inc(&current->signal->sigcnt);  
> >
> > $ semind search signal_struct.sigcnt
> > def include/linux/sched/signal.h:83  		refcount_t		sigcnt;
> > m-- kernel/fork.c:723 put_signal_struct 		if (refcount_dec_and_test(&sig->sigcnt))
> > m-- kernel/fork.c:1571 copy_signal 		refcount_set(&sig->sigcnt, 1);
> > m-- kernel/fork.c:2298 copy_process 				refcount_inc(&current->signal->sigcnt);
> >
> > It seems to me that the only way is to use __refcount_inc and then compare
> > the old value with REFCOUNT_MAX
> >
> > Since I have not seen examples of such checks, I thought that this is
> > acceptable. Sorry once again. I have not tried to hide these changes.
> 
> The current ucount code does check for overflow and fails the increment
> in every case.
> 
> So arguably it will be a regression and inferior error handling behavior
> if the code switches to the ``better'' refcount_t data structure.
> 
> I originally didn't use refcount_t because silently saturating and not
> bothering to handle the error makes me uncomfortable.
> 
> Not having to acquire the ucounts_lock every time seems nice.  Perhaps
> the path forward would be to start with stupid/correct code that always
> takes the ucounts_lock for every increment of ucounts->count, that is
> later replaced with something more optimal.
> 
> Not impacting performance in the non-namespace cases and having good
> performance in the other cases is a fundamental requirement of merging
> code like this.

Did I understand your suggestion correctly that you suggest to use
spin_lock for atomic_read and atomic_inc ?

If so, then we are already incrementing the counter under ucounts_lock.

	...
	if (atomic_read(&ucounts->count) == INT_MAX)
		ucounts = NULL;
	else
		atomic_inc(&ucounts->count);
	spin_unlock_irq(&ucounts_lock);
	return ucounts;

something like this ?

-- 
Rgrds, legion