Re: Alloc refcount increments to fail

[Matthew Wilcox <willy@infradead.org>]

On Thu, May 02, 2019 at 04:46:44PM +0100, Al Viro wrote:
> On Thu, May 02, 2019 at 08:26:21AM -0700, Matthew Wilcox wrote:
> 
> > +/**
> > + * refcount_try_inc - Increment a refcount if it's below INT_MAX
> > + * @r: the refcount to increment
> > + *
> > + * Avoid the counter saturating by declining to increment the counter
> > + * if it is more than halfway to saturation.
> > + */
> > +static inline __must_check bool refcount_try_inc(refcount_t *r)
> > +{
> > +	if (refcount_read(r) < 0)
> > +		return false;
> > +	refcount_inc(r);
> > +	return true;
> > +}
> 
> So two of those in parallel with have zero protection, won't they?

We check that we're only halfway to saturation; sure we might go a
few dozen steps from INT_MAX towards UINT_MAX, but I have a hard time
believing that we'll get preempted for long enough that we'd get all
the way to UINT_MAX by unchecked increments on other CPUs/threads.