Re: [NAK] Re: [PATCH] fs: Optimized fget to improve performance

[Will Deacon]

On Thu, Aug 27, 2020 at 03:28:48PM +0100, Al Viro wrote:
> On Thu, Aug 27, 2020 at 06:19:44PM +0800, Shaokun Zhang wrote:
> > From: Yuqi Jin <jinyuqi@xxxxxxxxxx>
> > 
> > It is well known that the performance of atomic_add is better than that of
> > atomic_cmpxchg.
> > The initial value of @f_count is 1. While @f_count is increased by 1 in
> > __fget_files, it will go through three phases: > 0, < 0, and = 0. When the
> > fixed value 0 is used as the condition for terminating the increase of 1,
> > only atomic_cmpxchg can be used. When we use < 0 as the condition for
> > stopping plus 1, we can use atomic_add to obtain better performance.
> 
> Suppose another thread has just removed it from the descriptor table.
> 
> > +static inline bool get_file_unless_negative(atomic_long_t *v, long a)
> > +{
> > +	long c = atomic_long_read(v);
> > +
> > +	if (c <= 0)
> > +		return 0;
> 
> Still 1.  Now the other thread has gotten to dropping the last reference,
> decremented counter to zero and committed to freeing the struct file.
> 
> > +
> > +	return atomic_long_add_return(a, v) - 1;
> 
> ... and you increment that sucker back to 1.  Sure, you return 0, so the
> caller does nothing to that struct file.  Which includes undoing the
> changes to its refecount.
> 
> In the meanwhile, the third thread does fget on the same descriptor,
> and there we end up bumping the refcount to 2 and succeeding.  Which
> leaves the caller with reference to already doomed struct file...
> 
> 	IOW, NAK - this is completely broken.  The whole point of
> atomic_long_add_unless() is that the check and conditional increment
> are atomic.  Together.  That's what your optimization takes out.

Cheers Al, yes, this is fscked.

As an aside, I've previously toyed with the idea of implementing a form
of cmpxchg() using a pair of xchg() operations and a smp_cond_load_relaxed(),
where the thing would transition through a "reserved value", which might
perform better with the current trend of building hardware that doesn't
handle CAS failure so well.

But I've never had the time/motivation to hack it up, and it relies on that
reserved value which obviously doesn't always work (so it would have to be a
separate API).

Will